one of the three plants has grown very slowly. he ...

The round smooth leaves on the sprouts are the cotyledons. They naturally turn yellow and drop off as the seedling grows the larger, serrated leaves. The cotyledons turning yellow is completely normal. The plant is alive and healthy so long as the serrated leaves stay green.

Morning, I see your day temp is reasonable but the low is a little lower than most people recommend. If you can bring them inside in the evening thatd be good. Also a small cup over them would help retain heat and moisture(the babies don't have to a many roots so they like warm and moist. If they can't come inside then perhaps a blanket or at the garden stores they sell stuff specifically for plants to stay warm at night. Your last plant didn't look completely gone, but if you give a little tlc I bet it can recover. I hope they turn around for you. I'm doing outdoor and I haven't moved mine out at night yet Because our temps at night are still between 41-52. Happy growing

I've experienced the same issue when using peat pellets; sometimes the seedlings are unable to break free of the mesh, other times the acidic nature of peat inhibir roots development. I've had to gently break the seedlings free of then and transplant to soil

Your night air temperature is a bit cold, i had this problem early on in one of my grows and it stunted the growth for a little while, bring the night temperature up to around 18/19 Degrees. Don't overwater either as at this stage the water uptake for them is not as much as you might think, Let medium dry out properly before watering too. Other than that the plants are ok. Hope this helps 😉

In the early stages it is good to keep your humidity high. While 50% isn't low, it may have not been enough at 75 degrees, to keep the seedlings from transpiring too much. I've seen other grows where individual humidity domes are used.