Well, we have been looking into this.
3 numbers are a lot rarer than 3 letter names but against this is the fact that they cannot be used for acronyms, such as company initials.
The values 'seem' to parallel exactly the prices of 3 letter names in that the prices seem to start at a similar level and vary massively according to its quality. Instances of sales are hard to come by but I would guestimate the following values:
A. Approx $1000 - 2000 for rubbish, meaningless or hard to recall numbers.
B. $2000 - $5000 for half decent numbers or small meaning (or lucky chinese)
C. $5000 - $10000 for decent number or numbers with significant meaning
D. $xx,xxx to $xxx,xxx for prime 3 letter combos such as 911 / 999 / 777 / 123
Maybe others can shed some light on actual sales. Personally speaking we have had an offer on 562.com (Long Beach , Ca area code) of $4500 which we turned down.
As with most domains it comes down to desirability and, if somebody wants it badly, what they are prepared to pay for it.
I think it is pretty safe to assume that values will rise or fall in line with the general prices of 3 letter names. So, if you see a 3 number .com in the very low $x,xxx and you believe prices are still firming then BUY it!
As for your specific question, I would put 121.com in category D .