Would it be possible to wire a universal 3" teleflex tach on a 35 Vanguard rather than replacing the tiny tach?

No.

Every outboard is equipped with a pulser connection on the charging system, and a marine tach is designed to hook to it (and read the number of pulses coming from the charge coils). A briggs charging system is not equipped with that hook up.

Alright thank you

Not sure but look into it more. I ran an autometer gauges on my Yamaha rhino for bout 6 yrs without any problems. Had to wrap a wire around the plug wire and it used the firing of the plug to give the output. Was very accurate. It wasn't 3" but 5 1/2". Black in color with a back light. I ratcheted this to the tiny tach because I wanted to be able to see the reading with a needle.