Hi bazooka!
Not too hard to do 3 tone camo without AB.Just takes a little longer to set up. You need your red-brown and green paints in spray can format to do this though.
For 'soft' edge scheme you need to take an old white t-shirt and cut it up into random pieces no bigger than 4" square and not really any smaller than your thumb.Place your Marder on a pedestal about chest high(while standing) so you can walk around and shoot paint.Or,you can use a turntable if you don't have space for pedestal method.
Place/arrange strips of cloth on Marder to your liking with exposed areas to be painted with your first color.(doesn't matter which is first).Start with only a few open spots as you only want to paint about 1/3 of the surface if you are doing 3 tone. Shoot the paint from about 1 foot away and use light,quick shots.Hit each area from differant angles.When done,let dry about 20-30 minutes and then carefully remove cloth strips so that paint is not disturbed. you can re-use strips after paint is completely dry.Repeat this for next color.
For 'hard' edge cammo,use Silly Putty or Blu-Tack for masking instead of cloth and shoot as above.
Notes:
Practice,practice,practice on old hulks untill you are confident.
Any cloth is OK as long as it is lint free.Terry cloth towel is a no-no.Should be light weight material with a tight weave to block paint where you don't want it.
HTH,sorry it's so long.
cheers!