Lets just assume its it is distilled water, how many ml phosphoric acid at 85% concentration to to bring pH 5.4 to ph 3.0 for the 1 litre of distilled water. I really just need a number to start with to get things going. I'll work backwards from that.
0.001M solution of phosphoric acid have pH 3.0, that means around 0.06 mL added to 1 liter of distilled water.
This is not true, as I have no idea why the water had pH 5.4 in the first place, then it is very likely that when you add that amount of acid to your solution (you may assume it is a single, small drop) pH will change by less than 0.1 unit, depending on the composition. So in fact you still know nothing.
If you can't afford pH meter, buy pH strips.