I don't know what breed of lambs you have, freethyme, but some breeds would be getting close to slaughterweight at 3 months, so should be ok. However I know these are your first lambs, so I assume bottle-reared, and the never grow as well as those able to stay on mum.
If you are not finding it a lot of work to lock them away at night then it probably won't hurt to carry on doing so - I am still shutting my orphans up at night but only because it's a routine and doesn't bother me to do it - and it's helpful to have the little blighters locked up while I do other last thing (and then first thing) things!