I went to cameron highlands recently and it get quite cold there at nights.
I would suggest you prepare medium level clothing for your 3 months old baby. further to that you can always close the window to keep warm.
Do monitor you baby body temperature so that neither your baby will either get too warm or too cold.
Aside from that do get an apartment with hot shower as it will be nicer during the morning.
btw, you can also buy clothing there at the night market or shops around that area.
ear muffs or cap are recommended if your baby tends to get cold ears.
Hope this helps.
Travelling with baby, What to prepare and look out for?