Naval superiority is the key.
Once you get even the slightest hint of an edge in naval superiority in the european theatre, take on the french navy and destroy it, then blockade a french home port, which will completely cut off their trade income and destroy them economically. They will cease to be any kind of a threat after this as long as you maintain the blockade (you dont even need many ships to maintain it once their economy is gone).
Then rinse, repeat with the spanish.
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