Does the 3-in-1 (or 4-in-1) tie up your boil kettle?
With the boil kettle being combined with the fermenter, and since boiling takes only a few hours and fermenting can take 1-4+ weeks, the question of equipment tie-up arises. Is the 3-in-1 (or 4-in-1) going to be really helpful, or will it be a nuisance if one wants to brew a second batch but can't boil in it for weeks while the first batch is fermenting and conditioning?
The first and obvious response is that it's not a boil kettle being tied up, it is a jacketed conical fermenter which is being fully used for the duration of the fermentation.
The second response is that if one wants to brew more beer while the first batch is conditioning one can use a second 3-in-1 or 4-in-1 fermenter. This isn't blatant self-promotion, the economics are there and it is cost-effective. It is true that you will need an extra heating element to brew multiple batches at the same time but the cost of an extra heating element in your second fermenter compares quite favorably to the cost of a separate boil kettle, chiller and all the supporting hardware.
The third response is that the additional 3-in-1 or 4-in-1 benefits of 1. perfect sanitation every time, 2. reduced brewing time and 3. significantly reduced space footprint should for most, considerably outweigh any potential drawbacks.