This is my 10cents worth ! I have owned two MDX's . For me anyway, it is the best vehicle I have ever owned. I love that there is plenty of power at any speed and of course, any midsized to large SUV gives you the better visibility and feeling of at least being competitive in size out there on the road.
I have two relatives who own the RDX and I recently spent time testing the RDX when researching for my current vehicle . The RDX offers the advantage of being a bit smaller and thus is more maneuverable in parking lots or in the city. I got interested when Acura gave the RDX the 6 cylinder engine. HOWEVER, that being said, I backed away from the RDX because after I had been in the seat after about 15 or 20 minutes, I started getting uncomfortable . I think they skimped on the cushioning,which is a deal breaker for me. The MDX seat is comfortable even after a couple hours in the seat. That is something to consider.
I spent some time with the Lexus RX. and I wanted to love it . I really did ! One of my girlfriends test drove the Lexus RX and loved it , so each to their own. The lexus handles beautifully and is a smooth ride. I just find it boring ! The Acuras are just more fun ! And that is just personal opinion !
Both Lexus and Acura will treat you like a Queen or King ! Lexus has prettier bathrooms ! LOL
One thing you might consider inquiring about - I bought a 2016 MDX and had two safety features added, as of course the two I wanted were on the accessory list ! But they are well worth it . One is a beeper that when you back up and get within a certain distance of something behind you, the beeping starts and goes faster as you get closer . It also " sees " things moving , like a car, that you might not see, and it beeps when you are in Reverse. If you can get it, well worth the money. The other is a sensor that alerts you to a car in your blind spot. I am not good about backing up and I am not good about seeing someone on my right side ! I nearly captured 3 cars in the time I had my first MDX and all on my right side ! If you get the RDX , that should be less of a problem . Maybe Lexus comes standard with those two features !
Good luck, you are looking at three great makes of vehicles ! They all get about the same mileage. I average around 21 mpg.