@Jayden Hamilton congrats, you're in an awesome position to be in so young! Ultimately it depends on what your long term goals are and your risk tolerance.
Below are some options:
1) I'm assuming you live in either the duplex or SFH? - if so, (and assuming you've lived there more than one year), I'd find another MFH (2-4 units) to purchase with an owner-occupied loan and use part of the $100,000 for a down payment. You could rent out your current place as long as it cash flows.
2) You could either cash-out refi or put a HELOC on your SFH, up to an LTV % of your comfort level. Since it's an investment property, the max you'd be able to get is 75% LTV, or $120K - $130,000 depending on what the appraisal comes back at. With that money you'd be able to smartly scale your portfolio with a few more properties that cash flow well.
3) Make sure you have a solid reserves fund set aside within that $100,000, both a personal reserves and reserves for your rental properties. The personal reserves would cover unforeseen personal situations (lost job, medical emergency, etc). The property reserves would cover unforeseen repairs, damage, vacancy, capex expenses (new roof, A/C, etc). I normally have 6 months of expenses in my reserve account for rental properties, however this amount or value properly varies widely amongst investors.
4) Take your time, and grow smartly - I'm in a similar situation right now where I have a large amount of cash that I'd like to redeploy into additional properties. You will have the urge to pull the trigger on properties quicker in order to get your money working for you. The key is to know what your criteria is, and not adjust your criteria to purchase a property just because you have the money to do so.
5) Find a mentor - you're in a better position than 95% of the investors on here when they started, but you still have a lot left to learn, so try to find something that you admire within the real estate investing and try to learn from their mistakes to avoid you making your own. Also try to participate in your local REIA events or BP meet-ups.
Hope this helps, congrats again, you have a solid base to build your investment career from!