OK, so I’ve been looking around at all the cheap deals in the UK for a Samsung Galaxy S3 and here’s what I’ve come up with.

Vodafone
Free phone, £36 a month on a 24 month contract. 600 mins, 1GB, unlimited texts.
Total cost over 24m = £864
Orange
Free phone, £36 a month on a 24 month contract. 600 mins, 1GB, 3000 texts.
Total cost over 24m = £864
O2
£99 phone, £37 a month on a 24 month contract. 600 mins, 1GB, unlimited texts.
Total cost over 24m = £987
Three
Free phone, £34 a month on a 24 month contract. 2000 mins (+5000 to 3 phones), unlimited GB, 5000 texts.
Total cost over 24m = £816
OR
You could buy the phone for £499 from Amazon, then get a SIM only deal from Three for
Free phone, £12 a month on a 12 month contract. 600 mins, 1GB, 3000 texts.
Total cost over 24m = £787
OR
You could buy the phone for £499 from Amazon, then get a SIM only deal from Three for
Free phone, £10 a month on a 12 month contract. 300 mins, 1GB, 3000 texts.
Total cost over 24m = £739
OR
You could buy the phone for £499 from Amazon, then get a SIM only deal from Three for
Free phone, £15 a month on a 1 month rolling contract. 300 mins, unlimited GB, 3000 texts.
Total cost over 24m = £859
Conclusion
Now if you know you will keep the phone for 2 years, you might be happy to go with the lock in contracts and save money up front. It will cost you more in the end. Assuming you can shell out now, there’s a real advantage in the SIM only offers from Three. First, you will get an unlocked phone and not be locked into a long contract. The final option is only 1 month at a time. Also, the Three deals are much cheaper. The final deal also gives you unlimited data.
Some people worry about the Three network. I can assure you, it’s much better than it was 10 years ago when it got the bad reputation. Don’t just trust me though, trust the independent government affiliated reviews (YouGov). They consistently rate Three as the best network, though maybe not for customer service…. but how often do you really have to call them?
From Galaxy S3 Forums
