How to extend the freshness of salsa? I often make salsa containing tomato, onion, garlic salt, green pepper, red pepper, lemon, and salt. I store it in a Glasslock container in the refrigerator. By the second day, the taste is still reasonable. By the third day, though it starts to get a strange smell and has a strange appearance, sometimes with white dots, perhaps from mold.

Is there anything I can do so that the salsa stays fresh for at least three days?
Acid is your friend here. You have some lemon, and tomatoes are acidic, but apparently that isn't enough. You should get at least a good week out of fresh salsa (mine lasts longer than that). Try adding a good shot of plain, distilled vinegar. Many recipes for salsa (including my own) include vinegar; add as much as you can without negatively affecting the flavor.

A bit of salt wouldn't hurt either, again, just to the maximum for good flavor.