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Hi! I'm a home groomer who's only groomed two dogs, who both happen to have the curly water-dog type coat, so take my answer with a grain of salt.
My understanding is that combs are an inexpensive alternative when you don't have a blade that will cut the right length. I used them for a while with my first dog - just snapped them over the 10 blade my clippers came with, and they did an okay job. I think they'd do better with an even shorter blade underneath - like a 30 or 40 - but I wasn't exactly doing show grooming so it didn't matter in my case. I think they're a great option for getting more length options without spending the money to have (and maintain) blades in each size, but I don't know if there's a circumstances where they'd perform better than a blade of comparable length.
Over time I've invested in proper blades for the lengths I use the most, and I like them so much better. Both easier to use, and produce a more even cut, in my experience. This is especially true now that I have my younger dog, who has much thicker hair. Combs just do not handle it as well as blades, even when I'm going for something as (relatively) long as a 3F. I also felt like I sometimes accidentally had hair get snagged between the blade and the comb and so they pulled more, but that could absolutely be my inexperience.
So based on my experience, combs are more of a substitute if you don't have and/or don't want a blade the right size. Having used them, I can't see anything they'd be able to do that a blade couldn't (but again, I've groomed a very small number of dogs with really only one coat type). Afraid I can't help you with lengths, though. I always have to look up what length each blade actually leaves the hair at, if I need to know, and usually just judge by the results I've seen. I suppose you could look up the mm the regular blades leave and compare that to the mm of the combs - like a #7 blade leaves 3.2mm, so the 3mm comb in your example would be close to a 7 blade.
My understanding is that combs are an inexpensive alternative when you don't have a blade that will cut the right length. I used them for a while with my first dog - just snapped them over the 10 blade my clippers came with, and they did an okay job. I think they'd do better with an even shorter blade underneath - like a 30 or 40 - but I wasn't exactly doing show grooming so it didn't matter in my case. I think they're a great option for getting more length options without spending the money to have (and maintain) blades in each size, but I don't know if there's a circumstances where they'd perform better than a blade of comparable length.
Over time I've invested in proper blades for the lengths I use the most, and I like them so much better. Both easier to use, and produce a more even cut, in my experience. This is especially true now that I have my younger dog, who has much thicker hair. Combs just do not handle it as well as blades, even when I'm going for something as (relatively) long as a 3F. I also felt like I sometimes accidentally had hair get snagged between the blade and the comb and so they pulled more, but that could absolutely be my inexperience.
So based on my experience, combs are more of a substitute if you don't have and/or don't want a blade the right size. Having used them, I can't see anything they'd be able to do that a blade couldn't (but again, I've groomed a very small number of dogs with really only one coat type). Afraid I can't help you with lengths, though. I always have to look up what length each blade actually leaves the hair at, if I need to know, and usually just judge by the results I've seen. I suppose you could look up the mm the regular blades leave and compare that to the mm of the combs - like a #7 blade leaves 3.2mm, so the 3mm comb in your example would be close to a 7 blade.
