It's all about finding the correct hand shape and feeling the keys underneath your hand. This applies to both hands, not just the left hand. I'm better at it with my right than left myself, but it's not really a problem to play a chord with my left hand for me. You'll get used to the feel of having your pinky and thumb spaced a distance apart that is the distance between the root note and the fifth of a chord (or an octave) soon enough, and it's easy to find the third in between with your middle finger (you should invariably play the third of the chord with your middle finger in the left hand - in the right sometimes you play it with the index finger, however, such as if you're playing an octaved root on top, or a seventh or ninth or something - depends on the chord voicing). It just takes practice to remember whether the third is on a white key or a black key. I couldn't tell you from memory, but it's just intuitive by now after all these years.