I just go some protein shake powder today...?

It says on it that sensitive individuals should not have in excess of 250 mg of phosphorus or they will get stomach upset.

I worked out that the 3 shakes i will be having per day contain about 389 mg ... Do i just hope i am no a sensitive individual, and if i am will the stomach upsets become less through time of my body getting used to it?

taking too much of it won't hurt you too much. phosphorous isn't anything dangerous.

if you get the upset stomach, cut back to 2 drinks. see what happens. if you're okay for a while, maybe knock it up to 3 and see if you're fine with it then.

most people won't be affected by it anyway.

Get the Protein Isolate is has less lactose and IS for sensitive stomaches. I have no trouble with it and i am milk sensitive.

I had the same concerns when I purchased my first batch. Like everything you put into your body, if it is the same thing over and over again, your body is a clever device and adapts itself to coping with it, thus eventually, your chances of getting stomach upsets are greatly reduced.
This is the same for the protein absorption that you will encounter when taking the shakes. If you haven't had a high protein diet before, start with 2 shakes a day, this is as the majority will be wasted and excreted in urine otherwise. You can test by using uristiks. Gradually up the intake (although, not above 4g per KG as this becomes harmful, and wasteful anyway) to around 2-3 g per KG or until you feel satisfied with your gym results. The partially permeable membrane will adapt to allow more of the protein molocules (which are slightly bigger than that of urea) through it and back into the body).
I would increase gradually, say every fortnight.