please tell me the difference

They're all very similar to each other and can often be used interchangeably, but there are subtle differences in meaning.

"Collision" usually refers to physical objects. When cars crash into each other, that's a collision. When non-physical objects, such as ideas or thoughts, are described as "colliding," than the word is being used in a metaphorical way.

A "clash" is what happens when you bring incompatible things together. These can be physical objects or ideas. Two colors that don't match are said to "clash." Two people who are arguing different viewpoints can be said to "clash." The word can also refer to a physical fight.

"Conflict" is similar to "clash." It also implies that two things are incompatible, causing tension. The difference is that "conflict" is a less agressive word than "clash"; it doesn't NECESSARILY imply physical confrontation, although it certainly CAN.