for(3; any condition; increment/decrement) here, how does 3 affect the code, is it allowed?

Answer 1

Syntax

for (statement 1; statement 2; statement 3) { // code block to be executed }

Statement 1 is executed (one time) before the execution of the code block.
Statement 2 defines the condition for executing the code block.
Statement 3 is executed (every time) after the code block has been executed.

You can omit statement 1

Answer 2

for(initialization/ Declaration; Condition; Increment/Decrement)
{
// for body...

}