Three levels would equal at most to 15% difference for breaking spell resistance. Furthermore even at higher levels spell resistance is usually not omnipresent. Ergo hardly a deal breaker. Level 15 means just barely getting one level 8 slot, two as specialist. Nice but hardly compensates for the idiotic damage bonus from SA unless you want to play CC machine/buff bot which is a viable alternative but besides the point. As I have shown applying metamagic feats make Scorching Ray already better than many other higher level damage spells and that is without SA. Triple SA would break the damage balance so hard that only braindead retards would advocate for it.
And again the rules make it clear that multiple instances of SA apply to full round attacks only. No spell attack is a full round action in addition to all the other stuff I mentioned. Summa summarum, still no argument to give Scorching Ray and Hellfire Ray three times SA.
Feel free to continue with your pointless rationalizations though.
10% difference because you would lose 3 caster levels compared to 1 you lose now. SR is on almost every high level enemy that is any danger. You get 2 castings of that spell of lvl 8 because you got Bond Object and that can restore one spell of any level. Also everyone plays a specialist mage or even a Thassilonian specialist.
And you are ignoring that fire resistance/immunity is most common trait, especially at high levels and you would be lagging 3 levels behind and that makes Hellfire Ray something you would get much later.
You sound like one of those people that makes a character for a campaign that starts at lvl 1 based on how good it is going to be at lvl 20. D&D/Pathfinder does not work like that, and neither does this game.
Also show me these rules that prevent SA with each of Scorching Rays? I checked the spell, it has no such limitations while Manyshot says in description that you can only apply precision type damage once.
You are the one doing baseless assumptions and it seems you are not even that familiar with base rules. Did you even play tabletop at any point?