If you’re really thinking of a 3rd monitor, depending on your financial situation, instead of a 3rd 27", consider a 34" as a centre monitor with the 27s flanking it on the sides. Also make sure that your video card has the capacity to drive that many pixels. (roughly speaking a 34" ultrawide is about 1 and a half 27" monitors)

Go to your local computer store and look at the sizes in person. 24" is fairly small by today’s standards and you may prefer 27", especially as you are approaching that ‘grey beard’ age and vision starts to go… :-)

Also look at the panel tech for whatever size you choose and read up on pros/cons of different panel types and what trade-offs you are willing to make to choose one over another.