I think this is catching a lot of people out (me included).

Travelling Zone 3-1 (or vice versa) from / to Forest Hill or Honor Oak Park

If you use Tube / DLR / Overground only, the Oyster PAYG fare is £2.90 peak, £2.50 off peak.

If you use National Rail + Tube / DLR / Overground (which you do if you travel via London Bridge National Rail station) the fare is £4.10 peak, £3.20 off peak - ie £1.20 / 70p more.

If you actually want to travel to say Canary Wharf, zone 2, then the difference in fare is even more extreme.

A zone 3-zone 2 fare on Tube / DLR / Overground is £1.40 peak, £1.30 off peak. If you were to do the same journey via London Bridge the £4.10 / £3.20 fare would apply because you have been through National Rail zone 1.

You can get all the fares from the TfL website http://www.tfl.gov.uk/tickets/. Hope this helps.