IMO TFE and TSE are miles ahead of SS3 (in terms of being a fun game, obviously not graphics and stuff like that). All 3 are good games but the old ones are so much better. SS3 just doesn't have the same classic shooter feel that the other two have.
Classic shooters had real level design and didn't constantly break out into 30-40+ repetitive enemy encounters.
Serious Sam has some enemy variety and strafe dodging but it evokes the classics the same way Call of Duty evokes CounterStrike.
Now I don't think being classic is bad or good but I think it IS important to note the design differences. Serious Sam is a different approach and should be noted as such.