Correct use of the word Bitcoin
From Introduction on the Bitcoin wiki:

Capitalization / Nomenclature
Since Bitcoin is both a currency and a protocol, capitalization can be confusing. Accepted practice is to use Bitcoin (singular with an upper case letter B) to label the protocol, software, and community, and bitcoins (with a lower case b) to label units of the currency.

So, the accepted usage in your example would be &quot;3 bitcoin&quot; and the &quot;Bitcoin protocol&quot;.