All of them can be used, depends on context:

a)To work as a team...
Para trabalhar como (uma) equipe...(geralmente virá com algo complementando,
veja ->)              as in:    To work as a team they merged they needed a plan.
                       To work as a team you need to develop your emotional intelligence.

b)Working as a team... (geralmente com algo antes, complementando.   Não iniciando frase).
       As in:
       "Their interviews touched on six topics: elements they considered essential for teams to work; elements they considered potentially leading to failure of the team; benefits of working as team members; drawbacks of working as team members; benefits of team teaching for students; and miscellaneous thoughts." 

       "It is good to see people working as team and is good to see teams releasing commercial modules..."

      "  "The students gained valuable experience in working as team. "Working on a team, you've got to be open to lots of ideas," said Matt Stinemetze, " ''

c) Work as a team      (also, it´s often comes with a complement, unless it comes as an order "Work as a team!"

      Example of "work as a team" at  http://ewh.ieee.org/r4/se_michigan/ems/mem.html
" Work as a team with the Event Coordinator, Greeters, Facility Liaison, and Speaker Host. Work any events you choose.    We need several volunteers for each seminar."

   Thus, it isn´t a case of right versus wrong.    

   You need to practice to get used to the structures.  Hope it helps.     

    PP.