It's a very simple task with spawn point outside the screen and some random or choose in the eventsheet
so choose which lane the object should spawn = choose (spawn1, spawn2, spawn3)
randomize e.g. time in between
randomize which object should spawn variable1 = random (0,1)
if varibale1 is greater then 0.9 then spawn bad stuff
there is a tutorial that should help you a lot https://youtu.be/9LDzIiHl97g?t=285 (here he starts using spawn points - but the whole tutorial is excelent)