I have been searching for a good 3d mmo engine for creating an online game, because I’m just an amateur programmer with small capacity to have broad ideas for making my own mmo engine for creating my online games, can game develop be used in creation of an online mmorpg/rts?(rpg & MBAG[multiplayer battle arena game])
No.
Game Develop have a network extension allowing to do some multiplayer game but it is rather complex to use and not fitted for a MMORPG. Creating a MMORPG is a very complex task involving difficult problems and needing a good experience in creating games using the network.
( I doubt you’ll find any easy and ready to use game engine to create a MMORPG by the way ).
You could try Eclipse Origins or NetGore for that.