it's just a probability problem. Compare with balls in a vase with a different color and you can pick 3 out the vase. You can make a matrix from this with the M+0-mass, M+1 mass and so on.
so first we can have Cl-35 or Cl-37, thus M+0 and M+2. Then the chance of having Cl-35 in your molecule is 3 times higher as Cl-37.
Then you can have only F-19 and in the last case we can have Br-79 or Br-81 with a ratio 1:1.
Now we can make a matrix with Cl horizontal and Br verical in order to masses:Cl --> 3 : 0 : 1
1 | 3 0 1
0 | 0 0 0
1 | 3 0 1
In this matrix we can see our isotopepattern with the peakintensity in our mass spectrum:
the M+0 peak has the mass of the original molecule is intensity 3.
Then the M+1-peak won't be seen, since there are no M+1-isotopes --> intensity = 0 (diagonal of the second numbers horizontal and vertical in the matrix
the M+2-peak has intensity 1 + 3 = 4
In the same way, we find a M+3 and a M+4-peakintensity of 0 respectively 1.
Thus in our spectrum we will see a M+0 to M+4 peak in the ratio:
3 : 0 : 4 : 0 : 1
In the same way it's for example also possible to calculate the peakpattern for example for ZnCl2 (you should try it