"First, second, third"

Translation: पहला, दूसरा, तीसरा

[GoWiP]

So, actually, why is this not written दूस्रा and तीस्रा?
Oftentimes 2 consonants next to each other, as here, get connected (बिल्ली, दोस्त, ...)

[srivassumit]

That's only done if the first letter is a "half" sound in this case दूसरा, both the स and र are full sounds

Unlike दोस्त where the स् is a half sound.

what you have typed would be something like "doosr", "teesr" instead of "doosra", "teeara" which is incorrect.