Complex sentence: Three years later, Gaines and editor Al Feldstein introduced horror in two of the company's crime comics to test the water, soon turning them and a Western series into EC's triumvirate of horror.
Split into simpler sentences: Three years later, Gaines and editor Al Feldstein introduced horror in two of the company's crime comics to test the waters. Finding them successful, the publisher quickly turned them and a Western series into EC's triumvirate of horror.