Effective emf of three cells in parallel, ε = 2 V
Internal resistance, r=?
External resistance, R= 5 ohm
Terminal voltage across the cell, V= 1.5 V 
Let, internal resistance of each cell be 'r' ohm. 
Effective internal resistance of each three cells in parallel, r= r/3
Therefore,
Total resistance of circuit $= \frac{\mathrm{r}}{3}+\mathrm{R}.$   
Now we have,
             $\boxed{\mathrm{V} = \frac{\mathrm{\epsilon R}}{\mathrm{r}+\mathrm{R}}}$

Terminal voltage, $\mathrm{V} = \frac{\mathrm{\epsilon } \times \mathrm{R}}{(\mathrm{r}/3) +\mathrm{R}}$
$\Rightarrow$  $1.5 = \frac{2 \times 5}{(\mathrm{r}/3) +5}$ 
$\Rightarrow$     $\mathrm{r} =5 \mathrm{\Omega }$