Indeed, most of the names proposed by you correspond to the preferred IUPAC names in accordance with the current version of Nomenclature of Organic Chemistry – IUPAC Recommendations and Preferred Names 2013 (Blue Book):
(2) 7,7-dichloro-4-ethyl-3,3,8-trimethyl-4-propyldecane (your edit is correct)
(3) see below
For compound (3), you have to consider the relevant rules for the numbering of locants for substituent prefixes:
When several structural features appear in cyclic and acyclic compounds, low locants are assigned to them in the following decreasing order of seniority:
(f) detachable alphabetized prefixes, all considered together in a series of increasing numerical order;
(g) lowest locants for the substituent cited first as a prefix in the name;
Note that Rule (f) takes precedence over Rule (g).
In accordance with Rule (f), compound (3) could be named as 5-bromo-1-methylcyclohexa-1,4-diene (which was your proposal) as well as 1-bromo-5-methylcyclohexa-1,4-diene since both names correspond to the locant set ‘1,5’. However, according to Rule (g), this example is named as 1-bromo-5-methylcyclohexa-1,4-diene rather than 5-bromo-1-methylcyclohexa-1,4-diene since bromo is cited first as a prefix in the name.