whoa i disagree completly.
I mix nutes all the time. I get a big ass bucket, pour about 4 different fertilizers in there (from 3 different companies), add some organic tea in there, and water.
they key thing is to pay attention to the ratios. 2 things. the plants need what they need, and they dont need any extra. what i mean is your 3.7-0-0 is fine, but id mix that, with some 5-5-5, and some organic teas, for vegging. then for flowering, i might grab that same 5-5-5 stuff, and add it to something more like 1-5-7 for example, just so they get more P and K. then you got all the trace elements, 16 of them (that we know cause harm if ignored) and companies never contain all 16. some have a few, others have a few more, and i mix those in to make sure i get the best blend possible.
DO NOT OVER FEED. thats the most important thing. and yea, i guess scrogg is right, you could blow your self up... so dont add nute to nute, add nute to water, then more nutes, then more nutes. keep your nutes separate (dont pour one nutrient into the bottle of another). PH test them to know what they will do to the water after its in there. and remember, just cuz its in flowering doesnt mean it cant use nitrogen, and just cuz its veggin doesnt mean some potassium or phosphorus is bad. the plants will gladly take some, but not much. know what they want, and when, and youll be ok.