Last summer my 3 year old (nearly 4 years old) was easily able to do 15+ miles on a trail a bike and was quite happy about it. She was very discontent in a trailer and would cry about having to ride in there. This year my 2.5 year old is unhappy in the trailer, but very content in a kid seat on my bike.

If I was in your situation I would have the 8 year old ride his own bike, have the 5 year old on a trail a bike and either put the three year old in a front kid seat or on a trail a bike too. That was everyone is seeing or peddling