There are ___ forms of linear equations in two variables.

Graph of linear equation in two variable is always a straight line because the two variable each have their own unique solutions and point on the graph. 

There are three forms of linear equation in two variables:

Standard form $ax+by+c=0$

Slope intercept form, $y=mx+b$

Where, m = slope

y = intercept

Point slope form, $y-y_1=m\left(x-x_1\right)$

Where, m = slope

$\left(x_1,y_1\right)$ is the point on the line.
 Therefore, 3 is the correct response.