j'm studyng for new rsi

 
want to make an indicator based on RSI but with one change.
Instead of simply close j want set the RSI on average of 2 bars of close less close of 3 bars ago.
j write so in TradeStation
RSI (average (c-c [3], 2), 14);
can someone help me to write for metatrader?
thanks
j rest available for exchange of material about technical analysis, I have several files(for free).
Reason: