Can i place a disc on top of one disc i previously place in the same spot?? If so... can i place my disc every time in the same spot in order to activate all my discs those turns
Yes it is possible, but it is not that beneficial.
All players will roughly get the same number of actions (either from the action spaces, or from the cards) the only difference in the number of actions depends on their choice of when to play their white discs.
Players have a limited number of discs, by placing three of them on the same space they are missing opportunities on other action types.
For example, by playing one disc on an empty action space, you will probably get those actions again two more times when other players will place on top of your disc (and they eventually will) and it will likely end up in the city council, helping with tie breakes.
If you can't wait and place on top of it again, you are getting two more actions right away, but now other players will be less likely to place one of their discs on top of your two (because they'll give you two actions against their one).
Now those discs are stuck there unless you keep playing on the same stack.
If you place the third disc, you will get the actions three times in a row. It is unlikely, though, that those two kinds of actions are so useful to you any more.
A winning strategy in Calimala is to try and do a little of everything rather than a lot of a few things: it's much better to try and have one cube in every place on the board than lots of cubes in a few places, (or worse, in your warehouses).