Su-15 has a strange issue: when seen in the the Encyclopedia, it has a correct Air-to-Air "maximum" loadaot: a set of R-8 (AA-3) AND R-60 (AA-8) missiles + fuel tanks.
BUT when the Su-15 is chosen from the flight deck ingame or in the mission editor, it can only take AA-3 OR AA-8 missiles.
In fact, Su-15 had 2 major versions.

Basic 1966 Su-15 aka Flagon-A (actually modelled in the game):
- Hardpoints: 4
2x under fuselage for 2x tanks;
2x under wings for AA-3 missiles;
- Engines: 2x 39/60 kN Tumansky R-11;
- Radar: RP-15 Oryol-D (NATO "Skip Spin") in a conical radome

Upgraded 1971 Su-15TM aka Flagon-F:
- Hardpoints: 6
2x under fuselage for 2x tanks OR 23mm gunpods OR 2x 500 kg bombs;
2x inner hardpoints for 2x AA-3-2 Advanced Anab missiles OR S5 or S8 unguided rockets;
2x outer hardpoints for AA-8 twin launchers (total 4x AA-8);
- Major wings and aerodynamics overhaul for a better low speed/TOaL perfomance;
- Engines: 2x 41/65 kN Tumansky R-13;
- Radar: RP-26M Volkov Taifun-M ("Typhoon") (NATO "Twin Scan") in an upgraded ogival radome.

In-game version looks like the Flagon-A, but I think it should be upgraded to Flagon-F, as Flagon-A with a pair of its AA-3's is next to useless. Or maybe both should be presented.

https://www.airwar.ru/image/idop/fighter/su15tm/su15tm-2.gif
https://warhistory.org/@msw/article/sukhoi-su-15-series-part-i
https://warhistory.org/@msw/article/sukhoi-su-15-series-part-ii
https://www.airwar.ru/enc/fighter/su15.html
https://www.airwar.ru/enc/fighter/su15t.html
https://www.airwar.ru/enc/fighter/su15tm.html