3-piece riser is doubtful the cause
I know this doesn't make the experience you've had any less frustrating but when my bottom limb cracked, about 1-1/2 years ago, I checked into the possibility of the 3-piece riser being at fault. From all the research I did I could not find even one instance indicating this riser design being at fault. Now, with that said, your particular case makes me wonder if there's something wrong with your specific riser such as how straight or perpendicular to the riser the limb bolt holes are or some other mechanical tolerance that's not right. Having gone through 3 sets of limbs and now needing a fourth set, I feel certain Joel will want the whole bow sent back so they can inspect it to a much greater extent to find the root cause of the many failures.
If this were happening to me I would also press Martin to pay for shipping this time, given the multiple failures this bow has had. I hope you keep us apprised of how this develops because this will definitely be a case that will show just how good Martin customer service is. I'm betting they will come through and take care of you and your bow issues in a manner that's very acceptable to you, the customer. They haven't gained one of the best customer service reputations by treating customers poorly.
2008 Martin Firecat Pro-X (29" DL, 66# DW), home-made TRG, new VEMs, DS Advantage/HHA Pro-5519 front sight, No-Peep Sight Eliminator, Limb-Driver arrow rest, B-Stinger stabilizer/dampener, custom upper STS, G5 Head-Loc quiver on custom mounting bracket. Gold Tip Velocity XT300 arrows with Bi-Delta 2.5" Sharkstooth vanes and Grizzly single-bevel broadheads, 498 grains at 262 fps (22% FOC)