Houses need proper roof venting.
Your builder will most likely have used minimum specs cheapest & least efficient to keep costs down.
(hate to say it but many houses were built with approved prints that were still not to code even when new.)
Look up roof venting ratios & requirements/recommendations.
They are along the lines of "good, better , best" for energy efficiency.
You really need to measure your house roof area in square feet (not counting overhangs).
If all the roof area is not connected each area will need separate calculations.
Pay attention to "upper & lower" venting requirements & spacing.
Yes, ridge venting will really help if done in conjunction with other venting methods such as soffit & gable end & "O-Hagin type" venting.
Yes, usually you should be able to find a 3 tab shingle to cut for ridge cap.
In some cases of higher end laminate shingles they only made colors that matched in "factory" ridge caps.
I was a Purchasing agent & estimator at a large roofing company for many years.
Good luck with your project, stay safe, remember to tie off your ladder.