The three-engine test bed was N9999W, the original PA-32 prototype. If I remember correctly they had the prototype's original 250 hp O-540 in the nose, and a couple of 115 hp O-235s in the wings, increased during the flight test program to O-320s. Recall that this was during the time when Cessna was selling a lot of C-337s, so weirdness in a light multi-engine airplane was not necessarily considered a bad thing.
The project evolved into the PA-34-180 with two 180 hp O-360s, and still with fixed gear and the PA-32 tail group. Performance must have been abysmal. The next prototype had retractable gear and 180 hp engines; then in succession came the larger vertical tail, and finally the 200 hp engines as on the initial production Seneca.