Following diagram shows the instrumentation amplifier using 3 Op-Amp
Instrumentation amplifier uses cross coupled difference amplifiers.
A1 and A2 are non-inverting amplifiers with their inverting terminal connected to $R_g$.
The input impedance of all the Op-Amps used is assumed to be infinite, therefore their input current is zero.
Thus current flowing through the resistors $R_1, R_g$ and $R_3$ is same.
The output voltage is given by
$V_0 = A_V x (V_1-V_2)$
Where Av is the overall gain which is given by
$AV = [1+2R_1/R_g]$ x $R_4/R_3$