Hello Ravi, 

Most IITs have 3 preference coloumns (or more)  under the specialization you choose. From example,  under mechanical, you can choose fluid and thermal mechanics as one preference and accordingly you can fill in the others. You do not need 3 forms for 3 preferences since there is space in one form for it. Since I haven't seen the form myself this year, I cannot tell you exact details but if there is space on a form for multiple preferences then you don't need to fill other forms for your multiple preferences. If a form doesn't have any space for you to insert your multiple choices, then you will need other forms (which is usually not the case with IITs) .

Hope this helps :)