I have three tomatoes up
-Yeah!!!

Anyways, I was wondering I have them in a tray w/a plastic lid over top under a light. How long should I leave them under the plastic dome?

Thanks!!

Take it off now hannah. The only purpose the plastic should serve is to help keep the soil from drying out faster until the seed germinates. Once they do germinate any covering should no longer be used or you will creating conditions that foster disease like damping off.

Yep, I'd say the same as John...
They can get too much moisture and dampen~off..

And don't forget to gently blow on them
and/or run your hand ever so gently over the top of them,
and bend them back and forth a bit..
*simulating the wind* so they grow stablizing roots right now too!!!!

This year I've decided to take the smaller tomatoes that come up in the same starter pot and transplant them. I'm not sure if they will survive the transplant shock when they are so small but I thought it was worth a try. I have an impossible time just plucking them out and throwing them away. Maybe this will work for me. So where I had one pot I now have three. There is still one pot with 2 plants in it but they are smaller so I'll wait a week or so and then transplant one if they both survive that long.

Thanks for your help-this is m first time starting tomatoes so I appreciate any info! Thanks!