Prepare to be asked 100 more questions!
First one - Actual three seconds or figuritive? If it can self suistain for a short time that may eliminate some causes (like no deadman in kind of thing) Likewise if it coughs & dies as soon as you let go the switch....
It's either going to be fuel or electrics related. I'll try to talk you through the thought process if stuff like that happens to me:
1) Engine cold or hot? If hot, post back, if cold, read on.....
2) How quickly do you put the fast idle / choke back? Not many outboards need choke for more than the time it takes to crank the engine, but I have one that won't idle without half choke for the first 5 mins..... (granted that's impossible with an electric choke solenoid, mine is a tiller control pull start machne) Might be worth trying to hold the fast idle lever up for a wee while longer. If that works, chances are your idle adjusters need tweaked or you got a blocked idle jet. If that fails, read on.
3) Any beeping noises as it happens / after the event? If so, dig your manual out, that will at least give you a step for a hint as to what to look for next (i.e what is an"instant shut- down", what that beep pattern means it thinks is wrong kind of thing. If not.....
4) This is where you need to get your hands dirty. Check all fuel lines for leaks & blocks (incl. the filter on the engine). If you have clear hose between your fuel pump & carbs, look for bubbles. Having said that I would doubt it would die after (literally) 3 seconds - most engines will run for a good 3 or 4 mins at idle when you disconnect the fuel., which is why I am thinking idle jets There is another live thread lurking here at the moment with a good description of checking your fuel lines.
If none of that (or any subsequent posts) help, post back & tell us
exactly what happened.....
