You have a lot of choice after installing SH3 and patching it to 1.4b.
In SH3, other than in SH5, you will like to play with various sub types, II, VII, IX, XXI can all be played and offered to your career when the date is historically appropriate . Keyboard shortcuts and GUI handling is much better than in SH5 in my opinion, and surely: faster and more convenient.
Much more Crew Management which matters also.
Not a megamod, but the best SH3 add-on is in my opinion
SH3 Commander (latest version: 3.2). Can be installed with any mod. Install it and see for yourself why it's so highly rated. A must-have IMO.
My favorite megamod is
WAC5.01, beautiful ports, beautiful bunkers, all stock grafics highly improved, and it contains a good added quantity of historical events. Good documentation (don't forget to also download the event list doc with historical background).
You need a fast computer (3.4 GHz, and 8GB RAM recommended; 6 GB RAM is enough but you need to install the '4 GB patch').
Atlantic battlefield with Baltic and Mediteranean patrols.
Recommended Widescreen mod with WAC: Manos GUI for WAC5, Ahnenerbe Widescreen GUI for WAC5.
Then, there is
GWX3.0 also called 'GWX Gold Edition' which shines with even more historical scenarios and a very good 500+ pages manual. Here almost all parts of the world are included for patrolling , even the Pacific region or the Black Sea.
I recommend this mod for getting started with SH series if you have never played SH3 before. Easy to install and get started, very stable. Here any low-medium computer will do, but you should have 6-8GB RAM, the 4GB patch must be installed during mod install.
Recommended Widescreen mod with GWX : Conus Widescreen mod.
LSH3-2015 is another super large mod, a lot of players will tell you it's the most eye-candy one and I agree , has also all sub interiors added, but I don't know much more about this one so I cannot judge.
I hope this is a start to help you
