1) Can you assist more than once in an adventure? (if there's 3 or more investigators in the same adventure)
If you can...
2) Can you assist more than once in the same roll? (the rules are especific between focus and assist, but not regarding multiple assist...)
You can assist or focus once after each failed dice roll.
So while you are trying to resolve an Adventure card, you will roll the dice several times. After each failure to resolve a task, you are allowed to Focus or Assist. You can only focus a total of once per Adventure. Each other investigator in your location can assist you once per Adventure.
So, if there are three investigators in your location you can be assisted three times. Bear in mind that you will not have many dice left in this case because you need to set aside one die each time you assist.
You can only be assisted by one other investigator per roll. That is, you may only place one die by the investigator who is assisting. You may not be assisted by more than one investigator after each roll.