If you want to create a simple 3D effect for your game, you should test sprite-layering as a simple and easy method. I am writing this for those who maybe didn't know about it. Tip: Its useful ^^

So, as I already wrote, the method is based on laying sprites above each other to create an 3D effect.

Step one: Create the sprites: To make the method work correctly, you should create the right sprites with sub-images.

For example, you want to create, let's say, a tank. So, start off by thinking  about how the different "slices" of a tank would look like if you slice it horizontally. What I mean by that is: 1st subimage: The tracks, 2nd subimage: The tracks with those "wheels", 3rd: Tracks and the lower hull plate, and so on. You will find a demo of what I exactly mean here;

So now, we need to draw them in the correct order.

So we do in a draw event:

for(i = 0, i < image_number, i++){draw_sprite(sprite_index,i,x,y-i,image_xscale,image_yscale,direction,image_blend,image_alpha)}

Thats it. Try it yourself!