They have the same meaning, but different usage.
جميع always comes before the word, for example:
جميع الناس = all people
أخذت جميع الأشياء = I took all the things
It's used the same way as كل
اجمع comes after the word, for example:
العالم اجمع = the whole world/all the world
And it have another forms like أجمعين so I think you should avoid using it now because it can be confusing.
جميعا is adverb, it comes after the verb.
ذهبنا جميعاً =we all went.
Sorry for my bad English.
Hope this helps.