You won't need to clear customs until you get to your destination country, only on the way back in.. so 3 hours may be enough.. but if you are changing air carriers in LAX for your international flight, you should definitely allow more time - because if the first flight is delayed and you miss your connection you are screwed. The first airline (and possibly the second, depending on fare class) owes you nothing if you miss the second flight.
When I fly two different air carriers internationally these days I plan for an overnight in between to be safe. There are just too many flight problems and delays now, as the system is at capacity and weather is worse in winter. If your entire trip is on the same carrier, they have to see you through, but 3 hours is still pushing it in LAX if there are delays.