I'm getting a huge difference in hashrate mining different coins from my GPU.
I'm getting 100 H/s for one. 550 H/s for other and 1600 H/s for another one.
Does that mean it's good for me to mine the third coin? (Assuming the coins have same price)
Hashrates on different mining algorithms are basically independent.
100 megahashes per second on SHA-256 (suppose you're mining BTC) is not the same as 100 megahashes per second on Scrypt (suppose you're mining LTC) and neither are the same as mining at 100 megahashes per second on Equihash (suppose you're mining ZEC), etc.
You need to run the profitability calculations on each coin based on how well your hardware performs on the algorithm that corresponds to the coin. There is no mixing.