What kind of fish oil?

Fish oil is made up of EPA and DHA.  If you want to reduce inflammation Metagenics 6:1 fish oil is great, as EPA directly reduces inflammation. Brain function is heavily dependent on DHA so a high concentrated DHA is great for mental health, cognition, and memory. General health support where you can reap benefits of both is where a traditional 3:2 blend fits in. 

3,000mg per day is a good amount. If you take less you are not likely getting the full benefits. It may be ok to take more but realize that fish oil is a natural blood thinner so watch how much you take if you are on a blood thinner. If you are not sure how much, ask a nutritionist, naturopath, or chiropractor. Often times labs or other simple tests can help with the appropriate dose.

Should it be a capsule or liquid? Personal preference. At Backbone you can get either. The quality must be high. It should be purified to remove mercury or other contaminants. If you have tried fish oil in the past and got the "fish burps" then you need the best quality. You can also try taking capsules and freezing them first. If that doesn't help then you may have a gut problem (weak digestion). 

For maximum benefits combine fish oil with 45 minutes per day of moderate exercise, healthy eating habits, chiropractic care, adequate sleep, multivitamins, and personal time to reduce stress.