I have to piping a multi base board heating two, or three tier element.. tubing,, my question is should I just put one termostactic valve, to control all of them or I have to put three of them in each tubing... thank you
You don't need more than one thermostatic valve per room.
You can actually put this anywhere in the room (where the pipes if more than one converge and then re-diverge) you choose provided there is just one heating feed pipe entering the room and one return pipe leaving the room.
Note: If the return pipe leaving one room becomes the feed pipe for the next room, etc. then there should be no more than one thermostatic valve in the entire loop.
There are a few systems where the feed pipe splits into a "passing lane" with no finned pipes to the next room and a "travel lane" that feeds the finned pipes in the baseboards. The two pipes reconverge just before entering the next room. Here each room can have a thermostatic valve in the "travel lane".
The good conscientious technician or serviceperson will carry extra oils and lubricants in case the new pump did not come with oil or the oil was accidentally spilled, so the service call can be completed without an extra visit.