I have asked this question to developers
MAMP & MAMP PRO. Here's what they said:
MAMP (PRO) 3.x does NOT bundle OpenSSL software but uses what comes bundled with Mac OS. If you update OpenSSL in Mac OS, MAMP (PRO) should automatically use the updated version. We will be including the latest OpenSSL in our future MAMP 4 release, which will be available soon. This will be a paid update.
UPD From comments (thanks to @lukemh):