I understand what Glen Alleman is trying to do here when he insists that you can pick all three of time, budget and scope, but he totally misses the point:
Put these estimates into a schedule, sequence the work. See what you get. Don’t like the outcome? Adjust One, Two, or Three of the variables and repeat the process.
Repeat until all three variables balance – fit into the acceptable norms of acceptance.
The big problem is, as they say, when everything is a priority, nothing is a priority. When push comes to shove, as will in any project, what will you give up on? Are you ready to go past the previously agreed upon timelines? Are you willing to allocate more money? Are you going to scale down your requirements for the release?
For example, if it is an “Industry Conference” you need to release for, you cannot adjust time. If you are a startup, your money is probably fixed. If you are trying to overthrow an established software player, scope cannot be changed. The whole point about TBS is that something cannot be compromised (at least not readily) and you need to give up on the others.
Same goes for “time, cost and quality”. The biggest danger about compromising on all three parts of the equation is that you may end up with something that is of low quality at a price nobody is willing to pay released at a time when no one is interested in buying it.
Projects that do such compromises go on for way too long, have their most important features gutted (because they usually take more time) and cost more money anyway. It is like what Jack Welch used to say about budgets. There is a compromise between the person who asks for it and the person who grants it, but neither are happy at the end. The asker didn’t get enough to do the work and the granter feels that they paid more than they wanted.