Hi!
Roulette and Blackjack involve absoultely no skill at all. (well you can card count in blackjack but still ^^ )
Poker is different though because you can control some variables. You get to pick when you invest money, with what cards and maybe even against who.
But i think the major difference that classes poker for me as not "gambling" is the fact that we dont play against "the house" we play against real people. Other people can make errors and we can take the expense. We can also perform "moves" and "bluffs" on certain players with stats. We can 4 bet bluff if we know they will fold 80% of the time to it.
So to sum up, there is a certain edge to be had in poker because of the fact that we play against real people and not the house. Because we play against people who can make mistakes, if we make less mistakes than them in the long run then we can be profitable.
But in blackjack or roulette in the long run the best you can hope for is breakeven because there is an even chance of winning the money. So you have to get lucky, to play profitable in the longrun
Edit: Although all of this, poker is still classed as gambling - but it can depend on what in your eyes is gambling
Regards,
-Jack