It depends in large part upon how you will be designing the universe objects, and how the fact tables should be joined.
Generally you will not be joining the fact tables together in the universe unless they are all at the same grain, in which case they can be joined on the shared PK. But that is an unusual scenario; more likely the facts are at different grains and rather than being directly joined, they are joined via the dim tables.
I'm assuming your Time dim has an FK in each fact table. You have two choices:
1) Use the one Time dim (no aliases) joined to each fact. Create three contexts and include one of the joins in each fact. In your BL, the time dim will be represented once.
2) Create two aliases for the Time dim, and join each one to each fact table. In the BL, you will have three Time dim folders.
Using the first method has the advantage of leveraging BO's ability to dynamically create separate queries. If you create a report with a Time object plus measures from each of the three fact tables, BO will create three separate queries. The results will then be re-joined in the report and you will see a single report block.
But sometimes this is not what you want. If there is little in common between the facts, then it may make more sense to use the second method. You don't have to worry about contexts, but you also don't get the ability to dynamically separate queries. If you create a report with objects from multiple facts, you will get an error.
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