cannot type tamil word "thou" "dhou"

After downloading software and installing latha fonts, I am able to type out all the Tamil alphabets, except the Tamil alphabet rows of "thou", "dhou". The problem I encounter is when the third dead key is typed out the first dead key goes off and the remaining dead keys' Tamil letter is shown.

Hope, I explained my problem. I do not know how to resolve the issue.

Can you help me?