Neither of these intervals is hard to play. Third and fourth have the small disadvantage that you occasionally need to play in a higher positions to get both notes on one string (e.g. when playing the third B-D we cannot play B-D as 1-3 on the A string, so instead we need to to play it as 5-3 on D and A string, which thus requires at least second position. This is something any non beginner violinist would be very used to.
The fifth does not have this problem, but usually string players do not like to play fifths as it is awkward to play: Fifths require you to stop two strings at the same position, which either requires you to bridge strings with one finger or to place one finger out of position. It is not particularly hard, but a bit awkward and string players tends not to like to do that very much.
So I’d say probably the fifth would be the "hardest". Regarding what’s "easy": It is hard to say. Both 3rd and 4th are quite easy to play. 4th as less cases where shifting to higher positions would be required, but then really what makes these intervals "hard" or "easy" is the how they need to be approached from the surrounding notes. A 3rds can be easier to shift by a step, as it leaves the fingers free required for the next one. On the other hand shifting a third by a third is more awkward.
So I think it is not really possible to generally classify one of these intervals as the most easy one to play.