I would just get a wide 11-32 cassette if you're worried - but first, do what hueyhoolihan says and try your existing bike without going lower than the middle ring.

I went from a 52/42/30 hybrid (flat-bar road bike) with a 12-25 to a drop bar racing style road bike 50/34 11-25 and I used to use the smallest ring on the hybrid big hills, but now I'm (mostly!) fine with the 34/25 on my road bike as a low. The lighter weight and better position on the road bike helped compensate. I have considered getting a wider cassette but I feel like I can really ride where ever I want on 34/25 as long as I'm not too tired.