My DD#1 wasnt 3 when sophie DD#2 was born. -SHe was 2.5. But we bought a three wheeler with a toddlerseat. When she got too big for it (around now, 3.5) we invested in a side by side stroller.
I was a bit like you , didnt want to buy a new pram when DD#1 maybe just as happy to walk, but i at least wanted the option of being able to have a seat for her KWIM?

At first she likes to walk......but towards the end of the shopping trip, her lil legs do need a rest. SHe's 4 in Sept....and still likes to sit in the pram occassionally.