In the US Channel 3 was preferred as it was in the midst of the continious 2/3/4 assignment. By lifting or lowering the carrier by 6 Mhz either channel 2 or 4 could be offered as alternative.
Another-Dave already answered the basic issue of avoiding collisions. The reason why it was channel 2/3/4 is due historic channel assignment (in the US, *1).
The following is all about the classic VHF low-bands (1/2).
Channel assignment during the 70s to 90s for VHF band 1 in the US:
Range VHF Chn. TV Chn.
44..50 MHz 1 (1) [Not used since the 1950s]
50..54 MHz gap - 4 MHz gap for [6m band]
54..60 MHz A2 2
60..66 MHz A3 3
66..72 MHz A4 4
72..76 MHz gap -
76..82 MHz A5 5
82..88 MHz A6 6
To start with, the channel assignments have shifted over time (*2). For example the original 6 meter gap was 10 MHz from 56 to 66 MHz, while 84 to 90 MHz was Channel 6. The gap got narrowed and with everything after moved to squeeze in another channel (*3) while widening the band for FM radio at the same time.
Important for consoles/home computers/VCR and alike in the 70s to 90s is that the FCC usually tried to space regional stations out to reduce interference between them. This didn't always work out in heavy populated areas, but usually either 3 or 2/4 was not assigned to a local (strong) station. Thus a modulator offering a choice to switch between 2/3 or 3/4 could usually solve any channel conflict.
A few devices offered even a three way switch (2/3/4) to allow the use of either channel within the group, but most manufacturers tried to save a penny, thus offering only two choices.
The whole issue about channel numbers on devices gets further complicates when looking at Europe(*4). US channels were 6 MHz wide, starting at 50 MHz, while European started at 40 MHz, occupying 7 MHz each (*5). So US 3 is located similar to European channel 2, while US 4 is European 3 (*6). So a European device marked as Channel 2 would in the US mean one has to turn to channel 3 :)) In addition in Europe UHF channels were often used instead - but that's different story (*7).
Now, in Japan TV did broadcast in the 90 to 108 MHz range (Channel 1..3), that's were FM radio is located in most other places. So Japanese market consoles / computers use frequencies usually not available to European and US TV sets.
And then there is (US) Channel 7 as Alex Hajnal reminded. It lies way above in VHF band III at 174 MHz. While channels 1..6 of band I are of different assignment around the world, 174 MHz is almost everywhere used for TV. In addition it's as well (in most countries) the first channel in this band, independent of spacing/alignment, thus ideal for world wide distribution.
Confusing? Yes, for sure, but in the end, one simply connected the console and turned the knob until some picture came up - if the result was poor, the switch got flipped to see if the other channel came better.
*1 - In Europe and other parts of the world it was similar, but different as the channels were assigned to different frequencies and of different size and arrangement.
*2 - Channel numbers and their logic are weird anyway, as they depend on convention, not physics. One have gone with frequencies, much like with Radio (and, if at all, use logical channels as in parts of Europe).
*3 - That's why they are names An in above table.
*4 - I'll spare the UK 405 line system using 5 MHz channels, as well as the special 1a and 2a channels.
*5 - Well, err , being Europe, there is much unity in doing things different. Beside the English 5 MHz system, there was the old French 14 MHz (awesome picture with 800 lines) as well as the new French 8 MHz system. The later also used in Eastern Europe and China but of course all with different frequencies and various gaps. For example the 6 meter gap is (mostly) observed in most parts of the world, except China and Ireland.
*6 - US 2 is located half way between European 1 and 2.
*7 - Most notably here European channel 36, which happens to be channel 34 in the US, thus despite the different channel spacing, the same modulator hardware could be used for devices designated for the US and Europe and thus essentially the whole world except Australia and China.