2351: Triplets

Unclued lights form three sets of three, each set related in a different way to a theme-word which is hidden in the grid and should be highlighted.
 

Across

1    Keeping on certain subject, I am an unusual scientist (13)
9    A court’s brief shade (4)
14    Mount paintings an artist submitted (6)
15    In position, mark hit (5)
20    Small holes in stone column you replaced (7)
21    Good money made, half in Oldie (7)
23    Risky amount to carry on horse (7)
24    Swimming organ, cold, leads to pains (5)
25    Recently winged (5)
26    Comparable condition, yet I lag badly (7)
32    Stupid-sounding right-winger in his element? (7)
36    Fighter KOs cross old farmer (4)
37    Murky liquidity in such a fund? (5)
38    A half-hearted mob in farmland (6)
39    Old poet’s limited range (4)
41    Rank one makes fast (4)
42    Like some wine? Assert with courage editor’s drunk (13, hyphened)
 

Down

1    Huge ship covered in blossom (5)
3    Page missing, wrote about a group of items (6)
4    Alps initially look vertical (5)
5    Philosopher you must admit almost genuine (7)
7    Expanse, some say, is better ventilated (6)
8    Value of equity in online business? (9, two words)
13    Candle for menorah fake? No end of confusion (7)
17    Good new puppies are shiny (7)
18    Most economical way one gets in supply (9)
19    One that bites yokel taking off end of the jaw (7)
21    Grazer left in superb pasture (9)
22    Engaging king, too briefly attentive (7, two words)
27    DNA sequences jail ordinary European: him? (7)
28    Hoodwinks lawyer before hearers (6)
29    These sung at ancient drunken social? (6)
31    You may be lucky to have one thousand at meeting (6)
33    Cattle pest: ox bitten upfront (5)
35    Smashed piece, using spades violently (5)
 
A first prize of £30 for the first correct solution opened on 9 April. There are two runners-up prizes of £20.