An equilateral triangle has rotational symmetry of order______.

In an equilateral triangle 

In the image we can see that an equilateral triangle has rotational symmetry of order three.

When we rotate it by $120°$ we get same form of triangle which we have seen in Fig. 1.

So $\frac{360°}{120°} = 3$

Hence, the order of rotational symmetry in the given image is 3.