Hello, thanks for commenting!
By "simple", I meant that the game doesn't have any advanced mechanics, not that it's easy to beat. :P
To beat level 3, you need to place the $20 tower, left click it and set its target mode to "STRONG" and then place $10 towers as fast as you can and pray for the good RNG (towers can sometimes "miss" the target, I thought it would be a good feature, but it seems it's only frustrating).
The levels were kinda rushed, so I am sorry you're having difficulties beating them. Anyway, thank you for checking my game!