We have a 3 wheeler with todd. seat, and a side by side stroller. My younger two are 19 months apart, so DD#2 was only 19 months when Sam was born. I loved the three wheeler for when baby was newborn....she could lie down, sleep, whilst DD#2 could have a good look around. I do know what you mean about climbers etc,but seriously, how long are you going to be out with all your chlildren? I found quick ducks into the shops the three wheeler worked fine. I also loved the three wheeler (as unsafe as some people think) as it was easier to steer. Im one of those people that carrys 28 shopping bags on my arms, coffee in hand and handbag, all whilst pushing a pram - i loved the three wheeler for that, it was doeable.
As DD#3 got older (6mths) she wanted to look around, no longer would lay in the three wheeler. We had the side by side anyway, so we started using that. Does the job, seats recline etc if they do wanna sleep. But i dont think its as easy to steer.
I would hang onto exactly what youve got and see how you go.....