This comes up more often than you might think. Company 1 gave you an offer, Co2 and Co3 are potential offers. Ideally, you'd like the offers to arrive at roughly the same time, so that you can choose the best one. So your job now is to get the other offers on the table ASAP, and delay decisionmaking for the offer you have.
* ASAP, but no later than Monday, 8/29, tell Company One that you have two other offers pending, and you'd like until after Labor Day to give them your decision. They should comply. If they don't, accept the offer, you can always quit later.
* Tell Company Two and Three that you have an offer on the table. Ask if you can move up dates, although usually you won't be able to do this. If they ask, tell them the terms you've been given. This gives them an incentive to provide better terms, and you'll get points for transparency.
* Be willing to negotiate soft comp terms - things like telecommute privileges, flexible schedules, additional vacation, expense account, company car, better equipment. Hard comp (salary, bonus) is sometimes much tougher to negotiate.
* Make sure to value benefits and commuting costs, not just salary, to compare apples to apples.
The advice "A bird in the hand is worth two in the bush" applies. Be careful in your dealings with Co1, since that's the bird in the hand. They're within their rights to rescind the offer if you push too hard. Your process sounds pretty early with Co2 and Co3 though, and could take months to finalize. You're better off accepting Co1 and continuing discussions with the others. Timing is everything, and backing out with Co2 and Co3 if you must, won't be held against you if you're honest.