Are you doing circle/storytime? When I had five 12-16month olds we still did a story time every day. I sang a few action songs and read a handful of toddler books. They LOVED it, some wandered while listening but for the most part they tried to sit near me while I read.

Stickers were a favorite activity. Buy the bigger stickers from the dollar store and let them try and get them off. I had to bend up a corner on each sticker for the first while.

With 1 yr olds you are limited to some degree but you can still have a schedule of activities. Periods of free paly, periods where you bring out a special toy to have them play with (blocks, potato heads, cars etc) that bring them together to spend a few minutes on the same activity.

Have periods where you put on children's music to sing along too (this works wonders when they don't yet sing and you get sick of hearing just your own voice). It doesn't take much to break up the day (simple differences like music on, music off, story time, a special toy activity) and it is enough to vary the flow of the day for your sake!

Once the weather is that little bit warmer and you start going outside more then you'll find the day goes by faster as well since just getting them outside and back inside takes up a good chunk of your day!

For the 3yr old. Any idea why he might be delayed? It sounds like he hears you fine? The lack of language output and the trouble with balance could be a hearing issue. I'm not sure what to recommend there. Part of me would worry that if he is that far behind then hanging out with 1yr olds is not the best thing for him, as he would benefit from the modeling of older children. But, make the most of your time with him and try to really work on the crucial skills he'll need for kindergarten. If he is far behind all around then I wouldn't worry too much about letters and number, I would likely rather spend my time working on his speaking, his expressing his needs (asking for water, food, the bathroom etc). Many children in kindergarten don't yet know their letters and numbers and lots of time is spent on this in kindergarten...but most children are able to speak in sentences, go to the bathroom with minimal help and express their needs.

If crafts isn't his thing then simplify it. Put paper and glue out and let him glue a variety of materials down (pompoms, scraps of paper etc). Let him use markers, let him paint etc. Structured crafts take a lot of patience and require the ability to complete multiple steps in a certain order. It sounds like he is far from ready for that. Let him explore some art materials without instruction...he free exploration may peek his interest, the more time he spends exploring the more he'll start to like it and the more he'll start to do and soon enough he will be ready for 'crafts'.

Have his parents taken him for a language assessment? There is likely a local group that does free assessments they will give the parents a lot of activities to do with him to help him along and they will flag him for any supports he may need.