How many 虾 are 三只半虾？
三只虾：3 prawns

good

半虾：half a prawn

Wrong. It should be 半只虾

一半只虾： half a prawn

Wrong. See above.

三只半虾：3 and 1 half prawns?? or 3 half prawns = 1 and 1 half prawns??

Three and a half prawns.
Chinese grammar wiki has a page about Expressing half with ban.
The structure &quot;Number + Measure Word + 半 + Noun&quot; means &quot;Number and a half nouns&quot;