I think for a solid third rod, I'd want a spinning combo. You have the heavy weighted single hook baits well covered with the first two rods. Just about every size you'd ever want to use. A good spinning rod in a medium power/fast tip would provide the ability to toss weightless plastics, top waters, inline spinners, jerkbaits, handle some drop shot rigs, shakeyhead rigs, and even work with 1/4 oz. crank baits to some degree. For this, I like a nice, crisp feeling rod with decent flexibility. Not too stiff but not too whippy either. IM7 or IM8 graphite is an example of the type rod I mean.
Spinning reels are generally cheaper than a comparable casting reel so cost could be kept down too.
Ideally for me, crank baits belong on casting rods, but also ideally, the rod is a specificly dedicated cranking rod. That might be spending too much right now for a single purpose rod. So you could get by with them on the spinning rod in order to gain all those other functions. You might even try them on the Shock rod also. I've heard those rods have a fairly flexible tip that might function similarly to a more flexible cranking rod.
Otherwise, if you aren't into the spinning rod idea, you may want to go with the same type rod, a medium with fast tip for a general purpose rod in a casting model. This rod can do cranks also, but easier (to me) since its casting gear, and can function for jerk baits well, and may do some of the other stuff that the spinning rod would do, but in my opinion it won't be as versatile. I see it as a "do all" rod, but only average in some areas, poorly in others, and maybe having two or three things it excels in, like jerk baits and light Texas rigs, and heavier inline spinners (a heavy inline spinner would be no more than 3/8 oz. on average). It's more like the first two rods you already have but weaker, in my opinion. I use no less than 12 pound line on casting gear, and go to spinning for less. It might be more versatile with the right reel and lighter line but if that reel is the Curado 50E then it might cost too much at the moment, like you said.
The line size of 12 or more on casting gear that I use keeps me from seeing this rod in the same light as a spinning rod of the same ratings. Also, I find that to fish drop shots or shakeyhead rigs, I do much better with spinning gear. Lighter line plus the way I hold the rod work better for me too. Also the spinning rods rated as M/F seem slightly softer than casting rods rated the same. I feel this helps with things like treble hook jerk baits and inline spinners. Inline spinners also are normally small in size, 1/8,1/6, and 1/4 on average, and do better on light line spinning rods over heavier casting rods and line.