Alright, I'm going to throw in a crap load of spoilers to explain why I think MGS1 isn't a necessary play to enjoy 2 & 3. Keep in mind this is my own two cents. and again... *SPOILERS*
Most people in the USA started playing Metal Gear around MGS1. That means they haven't touched Metal Gear and Metal Gear 2 (almost no one in America even played MG2). The things that happened in Zanzibar are pretty important, as well as the importance of Big Boss. MGS1 did a great job of filling you in through the codec and the cut scenes so you didn't have to know what happened in the minefield in Zanzibar or any of the previous backstory.
Introduce MGS2. It's pretty common knowledge now that Solid Snake is NOT the main character. The only thing I think is important about MGS1 that new players to the series will not catch is the reference to the bosses to MGS1 [which happens near the end. Yes that reference includes the cyborg ninja too]. That reference will be tied to the explanation in the end that ties the whole story together.
The important things to know in this story are explained in the first part of the game. Solid Snake is a hero. He has a twin brother. Their father is a legend. Metal Gear is a weapon. Snake beat it before.
With that in mind, you start the game with a whole new character. That's where the game messes with your mind. If you played MGS1 before, once you switched characters, you're playing this game with a new set of eyes. If you're new to the game, that's perfect. Because like the main character, you have no idea what's going on, and you believe snake is a legendary hero. The only times when MGS1 gets directly referenced is in the end, but if you have been paying attention to the story, you don't need to know all the details, minus a little reference about the bosses.
In MGS3, the game can stand on its own because it is the first game chronologically. You play as Big Boss, the antagonist in Metal Gear and Metal Gear 2. You will be playing as him, so you will presume him as the good guy. For me, I feel MGS1 isn't important to play, but MGS 2 is a wonderful supplement to this game. If you play MGS2, then you will approach this game with a more open mind. That ending in MGS2 will make you wonder what the hell happened? So you go back in time to dive deeper into the story. Why did big boss become bad? Who are the Philosophers?
The only game in the series where I feel playing through all the Metal Gear Solids is MGS4, but that's not included in the collection, so I'll skip out on that.