Easiest Mario
SMB3 is probably like one of the best games ever. I never beat it when I had it on the NES, I finally beat it when it was re-released on SNES.
I can only chalk up my inability to beat the game when it was originally released to my age, I was 5 in '88. I was like 11 when I finally beaten it on my SNES, and in retrospect it was the easiest of all the Super Mario Bros. released in the US on the original NES.
I noticed in other threads (one even stated that it's "Almost Impossible") people overstate the other games. SMB1 was the hardest; tricky jumps, limited lives, going from fire-powered Mario to small Mario in one hit (no in between phase), and the castles were hard, and the Hammer Bros. unleashed more of a fury in their hammers.
But SMB3 is easy; super easy to rack up extra lives, Toad's Houses, really helpful power-ups, the P-Wings, and the non-linear gameplay where you could avoid certain levels all together.
Despite the fact that SMB3 is the easiest in the original NES series, it's still a challenging game but not tediously difficult and all around good fun.