The bit rate used by Spotify seems a bit of a mystery but its supposed to be equivalent to 160kbps mp3. Lets then assume that its 128kbps AAC.
128kbps/8 * 60 seconds * 80 minutes * 30 days = 2,304,000,000 bytes or approx 2.3GB
If it was actually 160kbps then the above would amount to 2,880,000 bytes or approx 2.9GB
You also have to remember that Spotify is a peer to peer type application. Your PC will not only be downloading music but it will also be streaming it to other users so it will also have upload bandwidth requirements. Not sure if this can be restricted orr limited to protect your usage.
It will also not always get the data right first time round and there will be a general network overhead associated with using the internet which will push up the bandwidth requirement - not sure what it is but it may be in the region of 10 to 20%. It is probably quite high on a mobile network compared to landline based boradband.
So a 3GB allowance will in my opinion be way too little.
The 3 15GB offer is very competitive and they are the leading "dongle" provider in the UK, but their customer service is appalling. Everything seems to be done from India where the customer service reps get a script to read from which they are seemingly unable to deviate by even a punctuation mark. It took me an hour and a half to cancel a contract with them that I had not been using for a year (they gave me free minutes a year before but I never used the phone and the deal had run out). I had to exaplain that I didn't need it, that I had no friends at all (otherwise they said I should transfer the deal to a friend), that I didn't need a 3rd phone even if it was a good deal ( as it was still costing more than not having one, and I wasn't going to use a 3rd phone when I had two others and that it was just another thing to carry), and that I didn't want a call from a sales rep a month from now to see if I definitely wanted to cancel (I just wanted to cancel it now). It was the worst customer service experience I've ever had. Speaking to them was like speaking to someone from another planet - the culture gap between the UK and India is huge in terms of customer service.
Ooops, edit: Thought you were gonna listen for 80 minutes per day, not 80 hours per month. In that case the above figures should be: 4.6GB and 5.8GB ... clearly more than 3GB and not alloowing for any of the other factors.