Can three bright flashlights with No.7 batteries be directly replaced with 18650 batteries? It's very direct. You don't need any change. Thank you.

It depends on the bulb. Some light bulbs can and some can't. I have a strong flashlight shared by two batteries. However, because the magazine of three No.7 batteries is not the same length as that of 18650, no spare tube was provided when buying it, so at most, don't try to make it light, which will also affect the life of the lamp.