Hello. Yes, KNO3, KH2PO4 and K2SO4 will provide macros (NO3, PO4 and K).
By using KNO3, for each 10ppm NO3 added, will also add 6.3ppm K
By using KH2PO4, for each 1ppm PO4 added, will also add 0.4ppm K
So, if you don't have any sign of K deficiency using these salts, you probably may skip the K2SO4 dosing, or use just enough to provide roughly the same amount of NO3.
I suggest the website Rotala Butterfly Nutrient Calculator
to calculate how much you should add dry of each salt or add a few mL from a pre solution (I suggest to make a concentrated solution and dose by mL).
Magnesium is also consumed in a good amount along with calcium, so you should keep your water at least with a general hardness between 4 and 8 °dH (depending on your fish and plants) and a Ca:Mg balance of 4:1 ~ 3:1. So, depending on your tap water parameters (or using RODI water) you should also buy CaSO4.2H2O to dose along with MgSO4.7H2O.
In my tank I use NaNO3, KH2PO4 for nitrates and phosphates; K2CO3 and NaHCO3 for potassium and KH; CaSO4.2H2O and MgSO4.7H2O for GH. For NaNO3 and KH2PO4 I use separated concentrated solutions (easier to correct the amounts of each separately), and for K | KH | GH, I dissolve the salts directly into the new water of the water change.