Solution to 3D has been deliberately left unsolved and is to be answered only by a non-regular / novice commenter, with proper annotation. Those who have answered earlier in the week, please give others a chance.
Open for anyone to answer, if not solved by 1 PM.
ACROSS
8 Cart-horse carries god (4) THOR {T}
9 A towering Scot's arrangement without budgetary limitations (2,3,5) AT ALL COSTS {A}{TALL}{SCOT'S*}
10 Impel learner? Nonsense! (6) DRIVEL {DRIVE}{L}
11 Obtain one chance to receive an injection (3,1,4) GET A SHOT {GET}{A}{SHOT}
12 God! Free gold in passage! (8) CORRIDOR {COR}{RID}{OR}
16 Charge covering team leader in event arranged to raise funds for charity (4) FETE {FE{Team}E}
17 Chinese secret society's instruments for torture (5) TONGS [DD]
18 One meat rejected by nurse in China (4) AMAH {A}{HAM<=}
19 Flying machines Doctor No returns to Spain (6) DRONES {DR}{NO<=}{ES}
21 Emphasises, "School has locks" (8) STRESSES {S}{TRESSES}
23 Placing wood in ship, sleeps (8) SLUMBERS {S{LUMBER}S}
26 Large car part (6) BUMPER [DD]
27 Deceitful German lady head of department at youth centre gave loan (10) FRAUDULENT {FRAU}{De...t}{U}{LENT}
28 See 18
DOWN
1 Excitedly cheer at Rio for medieval driver (10) CHARIOTEER*
2 Cover extraordinary art and poetry (8) TRAVERSE {ART*}{VERSE}
3 What ladies at sea did? (6) ?A?L?D ()
4 String up husband found with a new girl (4) HANG {H}{A}{N}{G}
7 Mashed potatoes without peas for Prince of Bismarck (4) OTTO {pOTaTOes}*
13 Calls for some jewellery (5) RINGS {DD}
15 Tea with fruits containing energy for reptiles (10) CHAMELEONS {CHA}{MEL{E}ONS}
17 State wrongly, "Friend, the answer may be at the tip of your tongue" (5,3) TASTE BUD {STATE*}{BUD}
18 & 28A Mass production system one may see in school? (8,4) ASSEMBLY LINE [C&DD]
22 Discount on racket at last store (6) REBATE {RE}{BAT}{E}
24 Large ship's bird (4) LARK {L}{ARK}
25 Perhaps, legally proceed against Zulu canal (4) SUEZ {SUE}{Z}
Reference List
Learner = L, God = COR, House = H, Spain = ES, Husband = H, New = N, Girl = G, Saint = ST, Tea = CHA, Energy = E, On = RE, Large = L, Zulu = Z (NATO Phonetics)