It is ok to plan some of your activities around the toys that you have. Everyone could build towers with blocks and see how high they can build, talk about the colours they are using etc. with you giving more assistance (holding the tower so it doesn't fall over) for the youngest or give the baby a bucket with a few blocks in it and then do towers with the two older ones.

Set out cars and line them up in a row, count them, name the colours, discuss what job they do - ie dump truck or front end loader.

Sometimes the best activities are those using toys where we get the kids started and then we retreat or concentrate on playing with the baby and leave the older ones to carry on.