You could write it off as a fluke in February. When it happened again in March, people got concerned. Now stocks are tumbling a third time in 2018, and investors are starting to sense something has changed.

A smattering of 3-percent plunges may not make a bear market, but it sure is a break from the past, which saw only three such ruptures over six years. Sell-offs are getting more common — though no easier to withstand. Tech has been bleeding red, President Trump is railing at the Federal Reserve, and stocks that sat comfortably at record highs just three weeks ago have had just one up day in seven sessions.

For Donald Selkin, chief market strategist at Newbridge Securities in New York, it’s meant four times as many client calls and a bunch of frayed nerves. As one awful session followed another, the people asking questions went from clients to friends to family.

“It’s intense. I was looking at my screen and was like, ‘Really?’ ” Selkin said. “In February, there was a catalyst to blame. This time, it’s a structural shift.” That’s a common view among pros — that while this week wasn’t wholesale carnage, it’s more proof that a new era of volatility is upon us that is likely to last.

The Treasury will sell three-month bills and six-month bills on Oct. 15. They yielded 2.28 percent and 2.45 percent, in when-issued trading. It will also sell four week bills and eight-week bills Tuesday, and 30-year Treasury Inflation Protected Securities Thursday.

— Bloomberg News