Hi @Emma Lincoln
That's one large building (2,000+ sq. ft.) you have there. Sounds like a fun project. Here are some thoughts that come to mind.
1) You already have a commercial building in the eyes of the building code. 3+ units is commercial in the building code. 5+ units is only "commercial" in the lending world.
2) You absolutely do not need a licensed architect, but I agree with @Scott Radetich that you don't necessarily want to leave design in the hands of a contractor. Contractors typically don't think like designers.
3) Be prepared for a fire sprinkler system. This will be a must for the new basement units, and may need to be retrospective for your other 3 units since the new units are inside the same structure.
4) Design build may be a way to go. Architects tend to escalate construction costs due to the design.
5) Get ready for a fun permitting process with Portland. Since you're existing improvements are residential, there is a good chance you will need quite a few "building code appeals" to be approved. For example, basement ceiling height in the commercial code is 7'6", rather than the 6'8" for residential building code.
6) This will not be an easy project, but adding two units in Portland could quite possibly be worth the hassle.
7) If you have any intention to use conventional bank financing, do it now, while it's a 3plex so you're looking at 1-4 unit financing rather than commercial financing.
I hope this helps!