"I think you will find an ECG lead is a 2 terminal thing, the third electrode is there for noise cancellation"
2 + 1 = 3 (terminals - one +ve, one -ve, one grounded)
No wonder mathematical confusion reigns when considering ECG,
Consider Lead 1 + Lead 3 = Lead 2 ??????
Forget digging out the wide bandwidth storage scope "it is easy to measure on a scope" - Rojo . (dream on). I agree with felipespace " I got nothing" when using a scope. Significant CMRR, filtering and amplification is required, before you get anything readable.
I would go with Geoff's pragmatic approach, (which works for the rest of us mere mortals) cross check any suspect simulator or ECG module with a tried and tested CRT based Cardio-Rater or similar (even easier to do).