I would say wait till the baby is here and see how you go. Thats what I am going to do.. My youngest now will be 2 and a bit when baby is born.. My other son was 23 months when my youngest now was born but he just sat on the front of the pram or stood on the back..

Another option is to get your 3yr old riding a bike/trike.. and see if they enjoy doing that when you go walking