Steel is resisted by four types, not three. Water, fire, electric, and steel resist steel. Ice and rock are the only two types weak to steel. Electric is immune'd by ground, resisted by electric, dragon, and grass, and super effective against water and flying. Let's consider value here:
The four types that resist steel are very, very common. At least half of the bulky physical Pokemon are either water or steel which is very bad for a primarily physical offensive type. Fire is nasty since fire types also have STAB super-effectiveness against the STAB steel users which means that as long as the steel guys use said STAB, they're giving free switch ins to their natural counters. The electric one isn't so bad, but it's not great. Actual ice types are pretty rare; this is only giving you SE against Weavile (who was going to die to anything physical he didn't resist anyway) and Abomasnow (who isn't all that common). The rock SE is nice, but it's all you really have to work with.
Now let's consider electric. Ground immune is saddening, but at least most ground types are physical defense based while electric is primarily about special attacking (which means that any other special moves you have will likely help out greatly against grounds). Lati@s is the only dragon that actually resists electric. Grass types are really pretty rare. Being resisted by electric is, like with steel, not all too big of a deal. Water and flying are all over the place. Those are awesome SE.
The consider pairing with other types. What type does well against water, fire, electric, and steel? Ground is the only remotely passable type for this, and it leaves you with gaping weaknesses to a lot of specific dangerous Pokemon (Skarmory, Gyarados, and Bronzong say hi - also Moltres for epic win, etc.). Steel + psychic kinda works for getting neutral hits on a lot of things (watch out for Starmie), but that really only applies for Bronzong, Metagross, and Jirachi who have the right moves to use both types effectively and also just happen to have double STAB to play with to make up for the super effective hits that type combo isn't going to be getting. In general, steel doesn't work for a two type combo. Effective use of steel type attacks requires three offensive moves which isn't worth it when you could use other types and get your coverage with only two moves. However, consider electric. What helps against ground, grass, electric, and dragon? Ice! Ice + Electric IS a workable two type combo. Even with the more limited electric + grass or electric + water, you are beating out any of steel's two type combinations.
I hope this added some clarity to the issue.
