Answered by Ustadh Tabraze Azam
Question: 1) Should the dua to enter the mosque be recited before you step into the mosque or after stepping in? And the Dua to leave the mosque: should this be recited before stepping out when you’re still in mosque or after you have stepped out? 2) When drinking water: is the sunna 3 sips or 3 gulps?
Answer: Assalamu alaikum wa rahmatullahi wa barakatuh,
I pray that you are well, insha’Allah.
 It would appear that one would say the supplication for entering the mosque upon actually entering it. The same would seem to apply for the supplication to leave the mosque.
 It is a sunna to sip, and not gulp.
The Messenger of Allah (Allah bless him and give him peace) said, “Do not drink in one [gulp] like a camel. Rather, drink in two or three [sips]. Pronounce the name of Allah when you drink, and thank Allah when you are finished.” [Tirmidhi]
It is also recommended to pause twice or thrice whilst drinking to avoid [i] drinking in an undignified way, and [ii] making the container unpleasant for others.
[Munawi, Fayd al-Qadir; Khadimi, al-Bariqa al-Mahmudiyya Sharh al-Tariqa al-Muhammadiyya]
Please also see: Is It Permissible to Blow on Food to Cool It Down?
And Allah alone gives success.
Checked & Approved by Faraz Rabbani