As I mentioned in the comments, you're looking at the wrong aspect of armor. You shouldn't be looking at the percentage it reduces, but rather the amount of damage it actually prevents as a measure of how much longer that armor lets your survive.
Let's consider an example for a champion with 0 armor. Say he has 1000 health, just 'cause that's a nice round number. Now our champion gets caught in Garen's judgement, and starts taking 100 physical damage each second (at one hit per second). Clearly, our protagonist can take ten hits before dying.
But now, imagine our champion has the 50 armor you mention, meaning the incoming physical damage is reduced. Now (still with 1000 health) it takes 15 seconds for that bastard Garen to claim our champion's life. 50 armor extended our life by 5 seconds. (100 * (2/3) = 66.6 DPS; 1000 / 66.66... = 15 hits)
So our champion respawns, wises up, and buys 50 more armor. Now with 100 armor, we're sitting at a flat 50% damage reduction. Again we run into Garen, who's still dealing 100 base damage per second. It now takes him twice as long to kill us -- 20 seconds. Once again the armor has extended our life, again by 5 seconds. (100 * .5 = 50 DPS; 1000 / 50 = 20 hits)
Back in base, we buy another 50 armor. We're now at 150 armor, and 60% damage reduction. Enter Garen -- spinspinspinspinspinspin and it takes 25 seconds before our champion dies. Again, adding 50 armor extended our life by 5 seconds. (100 * .4 = 40 DPS; 1000 / 40 = 25 hits)
So even while the amount of damage reduced per point of armor diminishes (as you observed), the amount of time armor extends your life (or effective health) remains distinctly linear.
A major difference between 5s and 3s is how much lane control you need to win the game. To summarize: for 3s you want high dps champions with a lesser focus on team value, and in 5s you want high lane control champions with great team value.
In 5s you value champions that can hold their own lane well, because teammates will leave their lane open for minutes while they travel to help you. In 3s you value champions that can help other lanes and contribute lots to ganking, because traveling lane to lane takes very little time.
Champions like Heimerdinger and Soraka do well in 5s because they both have great lane control. Heimerdinger's turrets and passive heal allows him to push a lane very well and stay in good shape, meaning he doesn't need to leave the lane for a heal and spend a minute running back out. Soraka can heal herself and replenish mana, so she almost never needs to leave a lane.
However, in 3s they are less handy because they are squishy and susceptible to being ganked. They also provide little value when it comes to ganking in other lanes because they are not damage-per-second champions. To contrast Heimerdinger and Soraka, Tryndamere is a champion who shines in 3s. He can quickly travel from one lane to another by spinning over walls, and his dps makes ganks easy.
Another thing to note is that you require team composition in 5s, as any one champion can be focused down by 5 others at a time without a team backing them up. In 3s it's less of a problem because champions last longer in team fights as less damage is going around. Tryndamere in 5s can be crowd controlled by 5 other champions and he provides no aid to his teammates. However, in 3s he can solo two champions and likely win.
Best Answer
This map has not been updated in some time. So even if the files did still exist in the downloaded content on your machine, and you were able to activate it somehow, most likely it would cause errors during play.
Many walls, brushes, etc are different between the last released winter map and the current season map. I'm not sure exactly how league servers would deal with this, but most likely you'd be clipping inside walls as your computer would fight against the server to determine what is correct. At best you will have issues navigating the map and determining bushes, etc; at worst you won't be able to play at all until you revert the change.
That being said, I'm not familiar with a method to change the default map, and Riot (at the present time) does not support such modification of your client. AND the map would only be available for you if you were able to get it to work, not your teammates or opponents.