Identify the three major ways that the U.S. Constitution addressed the institution of slavery. Would you say the Constitution was a proslavery or...

There are really only three areas (in four locations within the document) in which the original (ratified) version of the U.S. Constitution addresses slavery. In Article 1, section 2, the document provides for how slaves are to be counted in a census for the purpose of determining how many Representatives the state is entitled to. Slaves are to count as three-fifths of persons for this purpose. 

In Article 1, section 9, clause 1, the Constitution addresses the migration or importation of slaves, instituting a limit of ten dollars per person for an import tax. This clause leaves the choice up to the individual state as to whether to allow the importing of slaves.

Article 4, section 2 prohibits “free” states from harboring runaway slaves, and requires that they be returned to their owners upon the owner’s request.

Article 5 prohibits any change in the prescription of how a census is to be taken until 1808. This essentially maintains counting slaves as 3/5 of a person until the way the census is calculated is changed, and this cannot be done before 1808.

The Constitution essentially makes provision to maintain the practice of slavery as it was at the time, and in that sense is neither pro-slavery nor anti-slavery. However, in mentioning slavery in the document, slaves are referred to as “all other persons” (in which case every person not a slave is filled into a previous category) or a person “held to service or labour.” The term “slave” is not used. This could be interpreted as an effort not to acknowledge slavery for what it was, or it could be just an attempt to use less offensive language.

The fact that Article 1, section 9 allows an import tax on people, relegates slaves to a commodity. I’m not sure that can be interpreted any other way. The fact that the Constitution prohibits changing the census process until 1808 could be seen as a safeguard to allow the country to settle into its new legislation before debating such a serious issue.

Thus there are arguments for calling the U.S. Constitution either “pro-slavery” or “anti-slavery,” and you’ll have to decide which is more convincing to you given the context of the writing of the document. I've included a link to the government archive page with the full transcript of the Constitution. The red text indicates portions that have been changed over the years, and links to more information about that.