In zone 6? The radishes should be fine to start hardening off- I got some that have been outside for several days now and surviving fine. Your lettuce might be OK depending on what kind it is, some are heartier than others, you can work on getting those hardened off. Your beans, way too early.

I use the 4 tier mini greenhouses. Got one set up for my onion sets and "scrap bucket"- a pot where I regrow stuff, currently cabbage and pok choi. I have insulation in the bottom of the greenhouse, and I'll wrap the whole thing in frost cloth if I need to to get through a freeze night. But I wouldn't trust it to keep my more tender seedlings safe overnight though temp dips below 45 or so.

It can really depend a lot on where you are growing too. For me, I have a sheltered southern niche outside my kitchen door that I can get away with using a mini greenhouse for the cold tolerant stuff and just having my seedlings out during the day when it's nice.... across my yard where the gardens are it's more open and still way too breezy and chill for me to do it there.

You probably can plant the lettuce soon, you might have to cover up from hard freezes. You should normally plant lettuce in April, lettuce can handle cool weather fine and quality is better when it is cool Spring.

Radishes can handle cold too, but you really shouldn't bother with starting inside, just plant direct since they grow so fast and the roots can be twisted. You can try to plant you radishes now and see what you get, or just plant some seeds outside anytime in April.

Beans, just dump those if they are standard bush "green beans" and plant seeds in the ground in a month when it is consistently warmer.

As to starting seeds too early, the question is how big you want to let them be when you actually plant them out. Planting too early means they're going to be BIG when you can finally plant them out. That means that you will have potted them way up to keep them from getting root bound. As noted, radish and probably lettuce can go out now. But you're going to have bean trees or bushes by mid-May. So if you're up to that, go for it. If not, them dump 'em.

In a cold climate, there is some value in planting seeds early for frost intolerant plants indoors that normally would be direct-sowed outside. That's so that you get an extra few weeks of growing season. For beans, that may make sense where you are.