Something's wrong for sure
, and before installing that spare alternator you need to find out why the other two alternators stopped working properly.
It is possible that a wiring
fault could be present wherein the connection between the alternator's output and the battery
is intermittent or broken.
It is also possible that someone turned off the battery
switch while the engine
was running, which would likely blow one or more of the alternator's diodes. Another possibility is an intermittent wiring
fault in the wire supplying field current to the alternator...at the switch, at the connection to the alternator, within the wiring harness itself, etc.
There could be other problems as well which would cause flaky operation of the alternator, related to the batteries, the regulator, or the wiring thereto.
It would be worthwhile (read much less expensive) to hire a competent marine
electrician or a mechanic
familiar with alternator wiring to find the problem. Probably wouldn't take more than an hours' time, and could save you a lot of $$$ and a lot of grief.