A lot of industries have books of standard schedules of rates and times for costing purposes. Where this is not available there are numerous time and motion studies from practically every industry that one could access on the net. Failing this, one could conduct a sample time and motion study to ascertain how long and how much effort a sample task would take. Only after exhausting the above resources would I opt for a guesstimate, which is only a rough opinion at the best.
F W Taylor in his studies proved that people would never willingly of their own accord operate at full capacity for any sustained period of time. Herzberg's theory of motivation and Maslow's hierarchy of needs indicates that people need to be motivated and the motivation factor has to exceed their comfort zone and trigger a higher need that causes the increased effort to be sustained. Pareto’s Law suggest that people will instinctively strive to work at 80 percent of their normal capacity and will when asked automatically make a 20 percent allowance in their performance estimates. In industry this level is accepted as being a lower 65 to 70 percent of personal capacity whereby manufacturing firms often class 65 to 70 percent of sustained effort as equating to 100 percent production target and are often willing to pay 30 to 35 percent bonuses in order to achieve 130 to 135 percent of production target or that is in fact a true 100 percent sustained employee effort from the production workers.
This clearly shows that there is no point in asking a person who has a vested interest in a work package to estimate the time or effort needed for its completion. If one does, then one cannot rely on the information without further scrutiny and or arbitrarily adjusting the figures to allow for the above peoples natural tendencies. This also raises a problem with antaganism when the project manager (through his/her experience) having adjusted the time down to a realistic level is then confronted by the worker saying the time is unrealistic, and through determined effort sets out to prove it.
Surely then, it is far better to seek this information from experienced people (internal or external) who are not going to be directly involved in that part of the project. Here, experienced project managers who have an extensive and diverse professional contacts list to call upon for advice have a clear advantage.
In any case, the adage 'archer without a target hits nothing' applies, every person working on the project should have a challenging deadline to work to that also addresses the problems associated with Pareto’s Law. Addressing this problem can be in the form of a time allowance when determining the 'effort time' associated with the task. Additional allowances may have to be made if one was to put the allowance on the 'task duration' where more than one person was working on the task (the multiplier effect). Of course when considering Herzberg's theory of motivation and Maslow's hierarchy of needs one would need to seriously consider the person doing the task to determine what type of motivator (carrot or stick) to use in order to get the job done on time and to the quality required.
Finally there are two pertinant points to make, the first is that project manager must insist, if not demand that workers time having been agreed and allocated by line manager for the project task is not encroached upon e.g if you have been allocated 2 hours of a persons time, you want a full 2 hour and not 1hr 55 minutes; and the second point, as a safety precaution, one could add an adequate contingency time element to the task (undisclosed to the worker) which if not used will cause smiles all around and bring your task in under the project time constraint.
Conclusion: Three-point estimating is a good idea but in my opinion, from the above discussion one should not gain the estimates from the people who have a vested interest in doing the task. Therefore get the estimate from suitably qualified people who do not have a vested interest and then sell the final time allowance to the task team members.