Q:

If three equal free masses $M$ are at rest at vertex of equlateral triangle of length $R$ at $t=0$.what is relation between position and time? 

I need a equation describing relationship between position of particle and time. You can assume only gravitational interaction between masses. 

A:

Newton’s second law tells us that the acceleration of a body of mass $m$ experiencing a force $\mathbf{F}$ is given $\mathbf{a}=\mathbf{F}/m$. 
Given that you are only concerned with the gravitation force, we can calculate it so: $$\mathbf{F}_G = G\frac{Mm}{r^2}\hat{\mathbf{r}}$$
Given the symmetry of the problem, you may want to consider a 1-D radial coordinate system.  You should only have to do the calculation once (as all three masses are experiencing the same thing). 