There should be a way to see directly if three discounts are being applied - first, check a rate with the DC and coupon codes going through the normal, non-EA reservation portal. Then check the identical rental, with the same codes, after logging in as an EA member.
If the two rates are the same, then the DC contract discount has replaced the EA discount. If the latter rate is lower, then all three have been applied, at least in part. (When I did this test, both rates were the same.)
It's entirely possible that for some EA memberships, one can do a triple piggyback. But I haven't seen it. (for the record, my EA membership isn't through Disney Club, since I'm not a member - just using the coupon in the sampler pack.)
At this point, this is more of a curiosity than anything else, since saving $5-10 really isn't worth the time it takes to secure it.