This is great news. Your product was validated by the market and your competition continues to validate your position as the best of breed. Way to go!
Don't think of these talks as acquisition talks, at least not yet. Use these inbound messages as an opportunity to develop your own business development / M&A skills, as well as a great way to increase your network.
Validate the person reaching out to you. Are they are gatekeeper or a dealmaker? The strategy or BD guy 3 years out of school is a great way to spin your wheels. At large public companies, deals get made or broken at the greater than or equal to VP level. If you are dealing with a gatekeeper, use them to get to the decision maker, but don't sink to much time into the relationship. Once you get to the decision maker, build a relationship without opening up the kimono completely...this could be a future hire, acquirer, mentor, connection, etc.
As far as protecting yourself, there's no great way. Diligence will bring your company to almost a standstill, you'll damage all your metrics in the process and your employees will notice that something is going on; that's just how it is. You need to figure out, generally, if you have a walk-away number and if you are in a position to get it. The best way to do that is talk to a range of people that might know (VCs, other entrepreneurs who have sold similar businesses, investment bankers, etc.).
Here's a secret they don't want you to know. If they are coming to you, in any serious way, they don't feel that great about their current position; so you shouldn't worry about them, just keep doing what you've been doing.
Lastly, if you have multiple serious acquirers, hire a professional team to get you through the process. Find a investment banker with expertise in your area and talk to their clients; bankers are expensive, but good ones are worth their weight in gold. Also, find an attorney you trust that has done this, with companies like yours and companies like your potential acquirers.
Good luck and if you have any questions, feel free to reach out. My info is in my bio.