To be honest, with three chainrings it’s unlikely you’ll need anything bigger than a standard cassette (if by standard you mean one with an 11-25, 12-25 or 11-28 range of sprockets).
If you’re really worried about getting up the climbs, you could mount a 12-30 cassette, but that’s as low as you can go with a road triple set-up that uses a 30-tooth inner chainring. Beyond that, you’re into mountain and touring bike groupset territory.
The main advantage of a triple chainset is that it gives you a wider range of lower gears to choose from by providing
you with a third chainring. With a small extra chainring you’re virtually guaranteed to get up anything – provided you
have the patience and puff to keep spinning.
The most common triple chainring configuration is 50/39/30, with the innermost 30-tooth chainring providing an emergency bailout option for when climbs get really tough. (It’s often referred to as a ‘granny gear’ – the thinking being that it’s so low that even your granny could pedal it!) As such, triple chainsets come in for a lot of stick (arguably one of the downsides of opting for one) but they make lugging a bike up climbs a lot easier.
With regards to cassettes, the wider the range you go for, the easier the gears you’ll have. But bear in mind that the wider the range, the bigger the jumps will be between certain sprockets, which can disrupt your cadence. You’ll probably also need a longer chain to get round them, which may mean using a long-cage rear mech to take up the slack.
Watch out too for your chainline as there’s more scope for running it at extreme angles with a triple.