Welcome to Subsim Brandmon!
SH3 multiplayer is merciless in one aspect:
If you and your multiplayer partners do not have the exact SAME installation of SH3 and/or additional mods... SH3 MP will not run.
For example... If you are using the 16km Atmosphere included optional mod... SO MUST EVERYONE attempting to share the MP game with you.
I say AGAIN for the sake of clarity (not to be mean) SH3 multiplayer is
merciless in this way. Each of you must be using the EXACT SAME FILES. Do not try to find exceptions to this rule.