Yes, in the sense it's a one bag policy on the base fare, but there are quite a few nuances on it. Firstly easyJet has the much bigger bag size:
easyJet cabin bag: 56 x 45 x 25 cm
Ryanair cabin bag: 40 x 25 x 20 cm
Wizz cabin bag: 40 x 30 x 20cm

Secondly all 3 airlines have various schemes relating to either bringing in a second bag in the cabin (usually involving the priority boarding supplement) or checking the bag. easyJet also has a scheme for checking in the first bag ("hands free"), where you can still take a "small item" up to 45x36x20cm and still take that on board. So if you have a standard way of travelling - in terms of bags - and you are disinclined to adapt that to LCC flying, then given the dimensions you will have various tactics in terms of baggage fees. So when you see a published are of 20€ or whatever, you will know that really means 40€ in your situation. But comparison is important, along with factoring airport travel costs, since a Ryanair 20€ fare can easily be more expensive than a 60€ Wizz fare, or indeed vice versa.

All 3 of these airlines aren't too mindless about sizes, as a very general rule, so they may turn a blind eye if your bag is a cm or two too large. But you cannot count on that, and they won't turn a blind eye to two decent sized bags on a single bag fare.