So..

There are three activity markers but only one activity/environment can take place at a time.... why?

Is it because a rumor and environment can take place at the same time... ok.. but then what is the 3rd for, some kind of special activity that can occur that I dont know about ?

Ive only played this game one time so far and we didnt get to finish but I love it still

They're general-purpose markers. You're right that generally it'll be one for a Rumor and one for an Environment. The third, though, could be any number of things -- sometimes an encounter card or Headline will cause something to happen in a space for a turn, and you might want to mark that.

Some Headlines can stay in play for one turn. Could be for them.

And the third one can replace the first one if you lose it...

Hugues said:

But what happens if you lose the second one

By rights, you only need the Activity markers for those Mythos cards that have the Activity heading and as far as I can see these headings only appear on Environment and Rumor cards. Since under normal circumstances you can only have the one Environment card and the one Rumor card in play at the same time then you should only need two Activity markers. But chances are there are certain situations where you may need an extra marker, what they are I'm not sure without checking every card in the entire Arkham Horror range.

Cool. thanks guys for clearing that up. Now I am hundred percent clear-minded on that subject.