The lesser the difference in the number of teeth for adjacent rings, the smoother and easier the shifting. Smaller differences are a worthy goal. A combination of 30-42-52, which comes stock from the factory on many trikes, is an excellent choice for all around triking, but not at all acceptable for the overland triker. Do not remain with this combination up front if you are heading out cross country! The mountais will spell your demise.
The 30 tooth small ring is far too high for negotiating serious mountain ranges with your trike fully loaded with touring gear, almost guaranteeing knee and foot injuries! And if you’re pulling a trailer in addition, the first big mountain pass will absolutely terminate your journey if your smallest ring is only a 30 … either that, or you’ll really mess up your feet and knees. A 26 tooth small ring is the largest any serious long haul triker should be running, and a 24 is clearly necessary if you are pulling a trailer. Run a 26 pulling a trailer and your knees will almost certainly be injured on long steep climbs after several 8-10 hour days, but a 26 works very well if not pulling a trailer (a 24 may be too low without a trailer).
I went through three different front end combinations, finally settling on the third as the sweet spot. My ICE Q came with a 30-42-52 Campagnolo, which was out of the question for serious mountain hills. I went to a 24-36-50 Sugino on my first cross country trek, pulling a trailer. The 24 got me up every hill, slowly, but surely. It was fine, but the 50 left me really wanting on the flat straightaways, where I could have easily picked up a few more miles per hour with a 52. So, I changed it all out again to my current 26-39-52 SRAM TruVativ Touro on my next trip without a trailer, and it was perfect, with the 13 tooth jumps smooth as silk.
On the old 24-36-50, I also had 152 millimeter crank arms, far too short for my long legs (I’m just over six feet tall). With the 50 tooth big ring and my 152 mm arms, I felt like I was just spinning way too fast for comfort as I attempted to pick up speed on the flat. With the current setup, the 170 mm crankarms and a 52 tooth big ring, I have plenty of torque (pushing leverage) and a healthy top end speed for most situations.
If you will be pulling a trike trailer, I encourage you to use a 24-38-52 chainring sizing strategy. The 24 will get a heavy trike and trailer up the longest mountain passes, while the 52 will allow you to crank up a good head of steam on the flats. Moving to a 38 for your middle ring means your chain will be making 14 tooth jumps on each front-end shift. This is only one additional tooth per shift over my recommendation of 26-39-52 for overland trikers not pulling a trailer. If your front derailleur and cable are well adjusted, this trailer crankset model should serve you well.