Finding games that are worthwhile to play is also a deciding factor. Don says a 20% improvement should not be overlooked. That is true however just playing 20% more at the easier count brings in the same return. 12 minutes more per hour on average. Be easy on yourself.
I believe the sweet spot is a good level 2 ace reckoned count such as the RPC (lots of two card cancels), Zen (great count for any number of decks plus better insurance correlation), and also Mentor could be added to the list.
As Don mentioned, the extra %.2 edge adds up to real money over time. But to the original question, a level three count does add more edge and profit, but at a diminished rate.
Which system to use is the perpetual question beginning counters inquire about. When I started I used the RPC back in 1993. After taking intermittent breaks for college, work, the military, and then regrouping, I now use Hi-Lo. A big reason for this is that it is quick to teach others and there is a voluminous amount of Hi-Lo specific data in print.
I like Stanford's answer on pp. 213, 214 in Professional Blackjack;
"It does not matter much which counting system you use, as long as you are using something that compares high cards to low cards."
But if you are genetically predisposed with strong and quick math skills, there is good reason to use a two or three level count.