Normal Aliens in the Meeting on the Earth

Read given data first!

Let's simplify the process with Venn diagram like below.

1. We know, 3 had all three unusual features.

2. 12 had two heads and three eyes, 9 had three eyes and four arms, 8 had two heads and four arms. Here, 3 having all unusual features too need to be counted in each of Aliens' counts above. 

So, Aliens having 2 unusual features of 2 heads & 3 eyes = 12 - 3 = 9.

Aliens having 2 unusual features of 3 eyes & 4 arms = 9 - 3 = 6.

Aliens having 2 unusual features of 2 heads & 4 arms = 8 - 3 = 5.

3. We know, 73 had two heads, 28 had three eyes, 21 had four arms.

Therefore, the number of Aliens having only 2 heads as an only unusual feature is 73 - (9 + 3 + 5) = 56.  

The number of Aliens having 3 eyes as an only unusual feature is 
28 - (9 + 3 + 6) = 10.

The number of Aliens having 4 arms as an only unusual feature is 
21 - (5 + 3 + 6) = 7.

4. So, we have 56 + 9 + 10 + 3 + 5 + 6 + 7 = 96 Aliens having at least one unusual feature. 

Therefore, the number of Aliens not having any of unusual features is 
100 - 96 = 4.