Triangular arbitrage is the simultaneous opening of orders for three currency pairs. The direction of transactions and pairs are selected in such a way that simultaneously open positions insure each other. Due to this, you can open quite large lots, and the risk is low.
Variants of triangles:
EURUSD – USDJPY – EURJPY
USDCAD – CADCHF – USDCHF
EURGBP – GBPUSD – EURUSD
AUDUSD – USDCAD – AUDCAD
GBPAUD – AUDUSD – GBPUSD
The orders that are part of such a triangle are in constant motion. The Expert Advisor analyzes the real prices at the moment and calculates what they should be. If these values differ from the calculated values by a given amount, the Expert Advisor opens 3 transactions.

Testing of multi-currency Expert Advisors is possible only in the MT5 tester. For MT4, testing is possible only on a demo account in real time.