I didn't start riding until my daughter was 9. That was just coincidence but it worked out pretty well.

I have heard of some parents who just do the toddler ride, stopping constantly for snacks etc. With 3 kids, and only 1 that fits in the trailer, that surely would be a challenge.

I know a couple who are both extremely active, with a 1-yr old, who take turns, bring the kid in some fashion (usually a jogging trailer for a run), or use a babysitter.

I know another mom who has 4 small kids and she is an avid gardener. I think that keeps her pretty active, and all the kids are involved in the gardening.

Keep asking the question. Somewhere there's a creative solution that will work for you. With 3 small kids, bicycling might not be for you, or maybe it is-- a bicycle built for 4!