Summing three lexicon based approach methods for sentiment analysis?
What you're proposing is some very simple form of ensemble learning.
You need to have at least a sample of labelled data in order to evaluate any method. Using this labelled data you can:

evaluate each of the three methods on their own
evaluate your idea of averaging the 3 methods predictions
if you have enough labelled data, you could even train a model which combines their predictions optimally (this would be full-fledged stacked generalization)