My standard deviation is 42BB/100, so I used that. You can find that stat in HEM/PT3 though. Here's what 3BB/100 over 10,000 hands looks like. Pretty good chance (80%) that you are in fact a winning long-term player, though how much of a winner is unclear. 20% of players winning at 3BB/100 with a 42BB standard deviation will in fact still be long-term losers.
The 2nd image is after 50k hands. Almost everyone who is winning at 3BB/100 over 50,000 hands is a long-term winner. 92% winners, 8% losers.
The 3rd is after 100k hands, and now all players are long-term winners though the true winrate can vary quite a bit.
If a player has a larger winrate, and/or a lower standard deviation, that player could be classified as a long-term winner with a smaller sample size.