Which is better crest 3d white or colgate optic white for preventing plaque and cavities?

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The same. They are basically the same. As long as you have a good diet with little refined sugar, and you brush regularly (3 x / day) and effectively you should be fine with either. If you want the best toothpaste for prevention ask your dentist about prescription grade toothpastes such as flouridex or prevident 500.
Neither. Choose a toothpaste that tastes good to you and has fluoride. Fluoride has been proven to reduce tooth decay and bacterial flora in the mouth that cause tooth decay and bad breath. Proper brushing with a fluoridated toothpaste is your best bet. You can beef this up with daily Fluoride rinses with something like act Fluoride rinse. Your dentist can rx a more potent Fluoride if needed.
Neither. The most important way to prevent cavities is to avoid those foods high in sugar content or sticky carbohydrates that promote tooth decay. Next would be to brush and floss after meals, and rinse out when that is not possible. Finally, see your dentist regularly for checkups and cleanings. Almost all toothpastes, rinses, etc. Have very little additional relative benefits. Prescription ones do.