hi my twins will be 17 months when i have baby no 4 and i have been struggling too.but i,ve been looking into the abc adventure buggy as you can get a single pushchair and twin seats on top .then when the twins walk i,ll still have a single pushchair .they is afew on ebay but i think i might get a new one ??660 as on ebay they go for about ??450-??500.hope this helps.