I’ll be very specific about what I think you should do, based on a variety of goals you have suggested. I will also tell you what you have said that I believe will be very ineffective for any goal.
If your goal is to get better at carrying heavy things, specifically for upcoming strongman events, there are 2 real options. Pwnisher’s suggestion of heavy sandbags, heavy rocks, or keg carries are all better options than dumbbells, because the difficulty of farmers carry implements is not necessarily the weight itself, but the awkwardness of a loaded carry. Dumbbells will not be heavy enough to be particularly effective, and they simply aren’t large or awkward. SO. If the overall goal is ‘getting stronger at carrying heavy things’, I suggest what pwnisher said.
If your goal is simply to improve grip strength, I wouldn’t do it with dumbbells either. There are 3 things I would do to replace your 3 farmers days. One would be a heavy barbell hold for 3 sets. You will use a weight that you cannot hold for more than 10-25 seconds. On the next grip training day, you would also do 3 sets, but a weight you can hold for 45-60 seconds. This should be very difficult, and will be far more effective at building grip. An alternative would be 1-arm hangs from a bar. These are VERY VERY difficult for most people. I can’t do more than 30 seconds per arm, and I have excellent grip strength. If you can’t perform a 1 arm hang, you can do both arms and still have an excellent training effect. But the reason I suggest 1 arm, if you can do it, is that it will build the unilateral strength involved in farmers carries.