Now what do I do??

They need to be put into individual pots, then grown on until they are big enough to plant out.

As PP says, prick out each seedling and put them in individual pots and leave them there until they have a total of 6 leaves. Once they have those, plant them up where you want them to grow. If you're growing one of the giant varieties, such as Russian Giant, you'll need to start staking them because they'll grow at a rapid speed and height. Don't forget to protect them against slugs. Slugs love tender young sunflower plants.

Oh heck! Sounds a bit high maintenance!

Ok, I currently have 3 small pots, each approximately 3.5 inch diameter, with 3 seedlings in each, well, apart from one which has 2.

The seedlings are getting quite long and leggy, I have some compost and bigger pots, and some canes, so now what?

Oops, just saw post!

Any particular size pot?

I would plant then into the ground now in a sheltered spot pref against a sunny wall about 2 feet apart, and stake them with your canes.
Water well and keep them well watered.
I have a couple to plant out too...

Grammaretto they cannot go into the ground, must stay in pots.

So, do I now put them into their forever pots, or just go up a pot size or 2?

When you pot them on, hold by a leaf, not the roots.
It's going to be cold over the next few days, I'd leave them until next week before repotting.

Oh I see. Then the bigger the pot the better. You need to put them in their forever pots about 12 ins across unless they are a dwarf variety. You'll need a lot of space and a lot of pots.
Alternatively you could repot into a slightly larger one and then give some of them away.

Ah, they're going to be patio pot plants then? Again, the size of your pot will depend on which variety of sunflower you've grown. If it's one of the varieties that grown to about 6 feet or so, a pot about 12" or 15" diameter should be big enough. If you've chosen one of the big varieties, like Russian Giant, you'll need the biggest pot you can lay your hands on! As with any plant, the taller it grows, the deeper it's roots will be so that it doesn't topple over. And pot grown plants need to be kept well watered, whatever you're growing. And slugs will still slither up your pots and decimate sunflowers unless you use a deterrent.

Thank you all!

According to the packet, they are Gelbe Riesen, Helianthus annus?

They'll grow to about 4 or 5 feet tall so a 15" diameter pot should be ok although you'll still need to support them a bit with a bamboo cane and a bit of string. Try and prop you pots up on little pebbles or stones to keep it from being flat on the ground and becoming waterlogged; they don't like wet feet! They need full sun, if possible Phoenix but will tolerate a bit of shade. And they'll turn their faces to follow the sun too!

Can’t you harden them off and bung them into the garden? Sounds too much phaffing for a few sunflower seeds. I get loads of the wretched seedlings from the bird food .

No, they can't go in the garden Whitewave , no beds currently.

Thanks for all the advice!

Chewbacca must it be only 1 seedling per pot? I have a few pretty big ones, and wondered about putting a couple in.

They will be outside in a South facing sun trap garden!

That type could grow to more than 5 or 6 ft.

I am going to put some of mine in pots but some of them are smaller varieties.

How large are your pots?

Variable, Callistemon some around 12" , some nearer 2ft.

Chewbacca many years ago we used to drive from the Channel ports to the south of France every summer and we'd pass huge fields of sunflowers on the way. It was fascinating seeing them turn their faces as the sun moved across the sky.
The French for sunflower is "tournesol." ?

Pots a couple of feet in diameter should be fine with 2 or 3 seedlings Phoenix. Smaller pots, maybe just the one seedling. They put out an enormous amount of root ball and will completely fill any size pot you grow them in. When you come to empty the pots, at the back end of summer, you'll be amazed at how dense their root system becomes; all from one tiny little seed! And the good news is that you can save the seeds from these flowers and you won't need to buy any next year! Winner!

You've chosen a smashing flowering plant to get going with phoenix; dead easy to grow, almost perpetual flowers until autumn and free seeds for next year. Just don't forget the slug deterrent!

Thanks Chewbacca much appreciated!

Have compost, need to root about in the shed for some slug pellets and some of that water retaining gel stuff.

Might pot them on tomorrow, but bring the pots in overnight as the forecast doesn't look too good.

Am I being over cautious?

No, not over cautious at all, it's a wise move at this time of year. Any young plant seedling is tender and, if nipped by even a light frost, it will turn their tender leaves black and shrivel them. That can set them back a couple of weeks, so best to avoid it if you have a greenhouse or potting shed to move them to. Or, if the pots get a bit too heavy to start lugging in and out at night, you can just cover them over with horticultural fleece or an old bit of net curtain works just as well, (but maybe I shouldn't mention that! ) and that gives frost protection too. By the end of this month the risk of frost should fingers crossed be over. ?

Mine are in the greenhouse but not potted on yet. I think I'll leave them for a few days and not disturb them until after the cold weather.
In fact, I'm pleased that they germinated at all because the seeds were old and out of date.

Mine are still in the greenhouse in tiny pots but they have suddenly sprouted and are about a foot high now. How did that happen? I only have 2.

Goodness, mine are about two inches!
They've got a lot of growing to do.

Mind you, I thought they'd never germinate, I bought these seeds in 2018 being sold off for 20p a packet.

I was given five seeds, three have germinated. They are currently in pots on my window sill, are about 4 inches tall but very leggy and bendy - already have a stake ( old kebab skewer!) for each one. Two now have a couple of little leaves as well as the original pair. I can't see them surviving much longer, one puff of wind and I suspect the stems will snap!