Assuming you are using a dosage calculated according to the package label (50-100ppm N), you are not really adding enough phosphorous to cause toxicity. It is important to measure nutrients in order to achieve the desired results. The label instructions seem very reasonable to me.
When in doubt with nutrient issues, be sure to always water with an excess volume whenever you feed, so that some extra water comes out the bottom and flushes any salts that may be building up in the soil. Just water with a sufficient amount (ie. reasonable excess) every time, and flushing is already built into the program.
A fertilizer labeled 9-3-6 should behave just like a fertilizer labeled 3-1-2 when it is diluted to the desired dosage (usually determined by N concentration). They are functionally the same, given the same basic ingredients in those relative proportions.
I have used fertilizer rated 7-9-5 and 24-8-16 (equivalent to 3-1-2 when diluted) for all my cacti at about the dose recommended on the Jack's package, with good results.
It could be a pH issue with the water, I suppose. What kind of water are you using? Is it tap water, ground water, purified water, or something else?
