If you bed them on good quality straw, and give them a bit of clean straw each day, that'll keep them clean and they'll munch on at the straw too, which is good forage for them.
Silage may make them a bit runny unless they have straw too. Some people say hay makes them runny but that's not my experience. Mind our hay is proper meadow hay, the grass rarely sees artificial fertiliser and then only a tiny amount.
Ad lib forage yes, but with young calves it needs to be fresh, and good quality, soft stems. Don't just fill the hay rack up and leave them to it for days on end, it's best to give them only what they'll eat twice a day so it's always fresh.
At three months they should be eating 0.5 to 1 kilo of 16% cake each twice a day. If your forage is good, that's a reasonable level to keep them on through the winter. Personally I wouldn't push them harder at this age as you are planning to keep them through to finishing at 24 months.