How can I optimize 3 variables in order to maximize the end product?
This area is known as black-box optimization: you have a function $f(x,y,z)$ where you have the ability to evaluate $f$ on an input of your choice, and you want to find $x,y,z$ that maximizes $f(x,y,z)$.  (Here $x$ is the decision threshold, $y$ is the maximum % gain per trade, and $z$ is the stop loss, and $f(x,y,z)$ is the amount of ending money at the end of a backtest with those parameters.)

There is a broad literature on black-box optimization, with many methods you could try.  See, e.g., Google Vizier, BBComp, and derivative-free optimization; there are many other resources as well.

One last warning: your current procedure is likely to greatly overfit.  If you get a rate of return of 10% on your historical data, that doesn't mean this is expected to give you a rate of return of 10% on future data; the future return rate will probably be far lower, due to overfitting.  If you want to estimate the likely future rate of return, I suggest you split your historical data into a training set and a validation set (e.g., 4 months into the training set, 3 months into the validation set).  Use the training set for the backtesting and to select the parameters.  Then, once you have picked a value for the parameters, you can measure how well it performs on the validation set.  To avoid bias, you're only allowed to use the validation set once -- if you don't like the results you get on the validation set, you're not allowed to go back to the training set and try to choose new parameters.

Personally, I suspect that 7 months won't be nearly enough.  There are financial behaviors that only occur once every few years (e.g., due to financial cycles that occur on a multiple-year horizon), so it's likely that optimizing based solely on 7 months of historical data could lead you to parameters that might perform badly when the situation changes.