The first few levels are not so difficult to pass, but starting from level 3, it's hard.
what rewards were given for completing the first three levels, could you specify?
The rewards are not great, the free spins are mostly. I think that the higher the level, the more expensive the rewards.
This is how casinos works, the higher you wager the higher rewards you may get. Free spins sounds like a small reward but someone may win big from it unless there is a small max winning allowed from the free spins. Just do not force yourself to reach high level because of the rewards. Play because you like to do it only with your own limitation.