1.Try to enter 4 numbers and submit, the form/field should
not accept more than 3 numbers.
2.Try to enter 2 numbers and submit,the form/field should
not accept less than 3 numbers.
3.Do not enter anything and see the result....it should
prompt to enter numbers.
4.Enter alphabets,special characters and check the result.
5.Enter integers(negative and positive),decimal numbers etc.
6.If the three numbers are of integer data type, try
entering float data type.Check with other data types also.
7.Enter very large numbers/small numbers and check.

Like this we can add many more test cases depending on the
Specifications or Requirements document.

test cases for biggest 3 numbers
1> 999 = valid
2> 9999 =  invalid
3> 99 =  invalid
4> 009 =  invalid ( as it is same as 9)
5> space =  invalid
6> alphabets / special characters =  invalid
7> negative number =  invalid
8> decimals =  invalid

the guys both answered is not correct as the question it
self says no boundaries of text box but asked as the biggest
of three numbers.
this is try to check the biggest of three no by if else
conditions checking each one by other thats all
if more clarity mail me at jagadishk.9454@gmail.com

First Compare 3 numbers if first one is bigger than other 2
numbers then 1 number is biggest else campare other 2 value
and check whether second number is grater or third no. is
grater. if 2nd number is grater than 3rd number then 2no. is
the biggest number else 3number is the biggest number.

I feel. Since code is already in place to verify biggest of
3 numbers we just need to input some combinations:

1) All three same numbers check wat is the result. Eg:
999,999,999

2) Two numbers same and verify result : Eg - 500,500, 400

3) Bounday Value Analysis Eg: 499,500,501