3 bids for each job:
I guess it is no exaggeration that 3 must be the magic number because that is how many bids you should get for each of your projects as well. You may find that the highest bid you receive is more than double your lowest. This is a good indication that you can find a deal somewhere in between.

Having 3 bids or more for each project does two things for you: first, if you only get two bids you may never turn up potential discounts or deals and you might not ever realize that you paid an inflated price and could have got the job done for much less; and second, it provides you with a backup plan, if for some reason your top choice bails on you, you have other subs to choose from.

The difference in bids can be huge, and it is usually wisdom not to go with the lowest bid just because it is the lowest. It may open you up to shoddy workmanship, and the added cost of having to have something redone if it doesn't pass inspection. If a bid is much lower than others, it can indicate that either the person lacks experience, or is not qualified to handle the project, so be careful. This precaution doesn't mean never go with the lowest bid. It means do your homework on them by following up on multiple (3) references, and ask probing questions during your interview. If after these precautions you feel they are not a bad hire, watch them closely rather than pay exorbitant prices. The trade off is usually worth it, if you know what to watch for.

3 subs as potentials:
You are going to use the subs you get your 3 bids from as backups in case things don't work out with your top choice. You might have to hire someone else day of because your sub doesn't show, so have at least 3 back-up subs and you are not going to have to pay for delays in construction.

3 prices for each material:
You will want to do price comparisons for at least all of your major purchases, not to mention your not so major purchases. This could mean big savings to you. If you get at least 3 bids or prices for each material you have a better chance of getting a good deal. The more price comparisons you do, the better idea you have of what you should be paying for each material.

3 of pretty much everything!
As you can see, in the construction business you want to check all your options, or at least 3 of them. So, if you ever wonder how much of something to get, or how many people to talk to for advice, 3 is a pretty safe bet. So, when in doubt, pick 3.