Project Euler #1 - Multiples of 3 and 5
Functions in C with no parameters should have a void parameter:

int main(void)

Variables should be declared/initialized on separate lines:

int a;
int b;
int c;
int d;
int sum3;
int sum5;
int sum15;
int total_sum;

Try to avoid single-character variable names.  They're very ambiguous and say nothing about the variable's purpose.  The only valid exception to this is loop counter variables.
Statements/blocks of different purposes should be separated to increase readability.  That would be the divisions, the sums, and the printf().
Your operands should be spread out to make the lines more readable:

sum3 = b * (3+999) / 2;

Initializations should be done as late as possible to help aid in maintenance:

sum3 = b * (3+999) / 2;

Remove int a since it's never used.