There are many ways to have three putts: some conditions are not under your control such as huge greens, or undulating very fast greens.
To avoid three putts don’t get down on yourself about the conditions over which you have no control, and focus on those ways you can avoid three putts such as:
Keep the shoulders, arms, wrists, and putter as a single unit.
A single unit with a light grip will help you develop, and maintain good natural rhythm, which is the same for every putt.
Make sure you have a well-designed forgiving putter and
Trust your line once over the ball. Without trust you are lost and the three putt will be lurking.
Frank & Valerie