if ((a==2) || (b==3) || (c==4))
printf("The numbers can form a triangle.\n");
In the above code if a equals 2 then you will print the message, regardless of the values of b and c. You need to test that all the values are equal, so it is a AND b AND c, thus:
if ((a==2) && (b==3) && (c==4))
printf("The numbers can form a triangle.\n");
You should als reduce larger values to see if they are in the same proportions as you are using here; for example [4, 6, 8], [12, 16, 20] etc.