I guess we just have to agree to disagree here. I think soldering and shrink tubing wires on car installations (when done properly) is much more reliable than t-taps, wire crimps, or wire nuts. Your point that a solder joint won't handle strain is well-taken, but I don't think any of the non-soldering options are much, if at all, better in that respect.

To answer the question about three-wire soldering: Yes, just twist all three together, coat the wires with flux, and solder away. Shrink tube the whole thing.