Until you break a spoke, the spoke-strength is not a factor in your previous wheel failure. Wheels are inside-out suspension-bridges that work on tension and the higher the tension, the stronger the wheel. Up to the point where you start cracking the spoke-holes at the rim. I suspect that the wheels that failed you last year had way too low of tension and that's the cause of the wheel failure. Take a look at this chart of spoke-tension at various spots on a wheel:


Notice that spoke-tension in dynamic conditions barely increases as the wheel rolls. But at the bottom it loses most of its tension. The hub effectively "hangs" from all the spokes outside of the load-zone at the bottom. The problem is, if you have too low tension, the 3-4 spokes at the bottom lose ALL tension and the rim is then very sensitive to side-loads. Such as the wobbling from pedaling and people moving around. This side load will then cause the unbalanced spoke-tension to pull the wheel into a taco-shape.

1. What you need then is the strongest rim possible, the Alex DX32 rim that Jeff posted is amongst the strongest. I use it on our tandem and my wife and I have bombed down hills at +60mph for the past 3-years without any issues. Even ran over a giant 25-lb possum/raccoon/ROUS of some sort and split it in two. Wheel stayed perfectly straight.

2. The 13/14g spokes will work for you just fine, that's the maximum that'll fit the 3-spd hub. Figure out the spoke-length you need for 3x lacing with this Damon Rinard - Spocalc programme. Remember, the higher the spoke-tension, the more load-capacity the wheel has before the spokes at the bottom lose all tension. You'll want to tension it up the maximum tension recommended by the rim-manufacturer. In this case, around 110-120 kgf all around since you've got no dish (130-140 kgf has been used with this rim for extreme-load conditions). One popular technique in finding that optimum tension without a tensionometer is to get the wheel perfectly true, the go around and tighten the spokes 1/4-turn all around. Keep on going until the rim starts to taco every so slightly. Then back off 1/8th turn and it should be straight again. This is the limit of what that rim can take and the tension is now optimized for maximum load-capacity.

3. Also use a fat tyre, 26x2.125" at pressure optimized for the load. Probably around 50-60psi in rear, and 30-35 in front for 3 people. The larger the tyre and lower the pressure, the better shock-absorption you have and the lower the peak G-forces experienced by the wheel.