If you you are factoring in expenses such as maintenance and capital expenditures, so your property is being kept up, you are doing your part to fend off depreciation.  

Based upon my limited experience with real estate  financing in other countries (4), this is an anomoly and it is more common to find mortgage terms that are a fraction of the amortization - requiring that the mortgage be renewed/refinanced at least once.Even if you were to lock-in a residential mortgage at these low rates, you should count on your future opportunity cost of capital (i.e. major CAPEx expenditures) to increase and build your financing and operation model accordingly.

What I mean by that is starting off by looking at your finances as if your goal was to go get a bank loan-  - Keeping a budget and a writing down where all your income and expenditures go - Check your credit on bankrate.com- Go to a bank and ask them what they would require to see if you were applying for a home loan (hint: tax returns for previous 2 years, W2s for previous 2 years, pay stubs for previous 2 months, a personal financial statement, bank statements)-You might get denied, but the whole point is to get organized enough so you learn about the process by doing.

There hasn't been an inspection on the property yet, as I am first waiting to receive a list of all the properties' capital expenditures and the year they've been installed/repaired (unless original). 

. $350/month over paying the mortgage does nothing for maintenance and especially CAPEX expenditures.

If you can get all of that to work on this property, then it may very well be a workable deal.Another thought, I would suggest doing more research on maintenance and capital expenditures.  10% monthly may be a decent guideline in residential SFR, but I would expect higher expenses in a triplex (think about it, 3 kitchens, more bathrooms, more doors, just more repairs needed in general, plus you're dealing with a lower economic tenant pool that can be rougher on the properties).Hope that helps.

I also factored 10% of rent for vacancy, repairs, and capital expenditures (all separately). 

They could, and some have in the past, decide to do some beautification of the property, that doesn't increase rent relative to the expenditure that you're required to cough up.In my opinion, you have to look well beyond the numbers of this deal and realize that the risk factors may not bring you the reward they should demand.If you haven't yet, start listening to the BP podcasts and you'll hear some of the stories and learn some of the lessons that others have experienced for free.  

Have you estimated capital expenditures and what are you accruing each month for such expenditures?

Looks like you missed capital expenditures and maintainence but there is room for that in the numbers your giving.