3 tubes is likely too much. Mastic One is as good as you can get. I use a can, but it is not over two tubes, more like 1.5 for a set. They are on tight. Yes, tape works. But you can do better.
Your tires are not setting on gluing. I KNOW some of this advice is a bit different, but try this.
Mount tires before gluing 1-2 days and pump to 100+ a few times. Notice seat. Some tire/rim combos are just not as good. But, you will see it then.
Glue time...
Do 1st layer on rim super thin. As thin as you can. Let dry. A day is fine. 3-4 hours may be fine.
Do base tape layer pretty thick on the sides. You need little/nothing on the center.
Before base tape glue dries pump tire to about 20 PSI and mount. Make straight. Then roll the wheel with 50# or so on both sides. But the wheel on the bike if you have to.
If you really want it set ... get everything straight. Leave 10-20PSI and wrap tire and rim with rope indenting tire.
Then pump to 120ish. Got to do this in about 10 min of mounting, while glue is still complaint. See pic. That will set the tire well. I only do this 30% of the time, but it works.
http://www.bikeforums.net/19288801-post19.html