I have a martin x2oo that I am trying to learn to shoot three under.I would think the tiller is probabaly not set up for this.Should I be trying to learn to shoot 1 over and two under?

No, you should not change unless you just want to. Just keep an eye on your arrow flight, you may have to slide your string nock up a little. Most archers that shoot 3 under are using bows that weren't tillered for 3 under. It's no problem.