Because I have an engine of 13 hp four stroke 1 cylinder that pulls about 0:20 T/W ratio. and is it possible to multiply the thrust by two? Using two more small engines which is 6.5 hp each on the both side.
Only if the 6.5 hp motor and propellers have the same thrust efficiency as the 13 hp one. As the propellers get smaller and spin faster, the efficiency usually drops and if you're getting, say, 50lbs of static thrust from the 13 hp one, you will probably get slightly less than that from two 6.5 hp ones combined. So instead of 100 lbs from the three, it might be closer to 90-95 lbs.
Then there is the weight of the smaller engines. If the total installation of two small ones is more than the weight of the single larger one, you're getting farther behind in terms of total power to weight.
If the objective is to double the power of an ultralight with an existing single nose mounted motor by adding two small ones on the wings, it might make sense if you can control the weight, but you will probably end up with something significantly less efficient than just replacing the 13 hp motor with a 26 hp one, or putting two 13 hp motors on the wings and removing the one on the nose (assuming all the weight and balance issues can be resolved).