The Tesla chief executive first exercised just over 2 million stock options Monday that were valued at roughly $2.5 billion at the day's close, paying around $13.4 million in exercise costs.

He sold many of those shares the same day to cover tax withholding obligations, according to the filings.

After selling less than 1% of his holdings Monday, he sold about 2% over the subsequent two days, the regulatory notices show. He sold around 4.5 million shares in total over the three days, shrinking the size of his stockholdings in Tesla even after exercising the options.

Mr. Musk, the world's richest person, over the weekend polled people on Twitter about whether he should sell 10% of his stockholdings in Tesla and said he would abide by their vote. Around 58% of respondents supported a sale.

Monday's option exercise and sales were made under a preset trading plan Mr. Musk established on Sept. 14, according to regulatory filings, almost two months before he raised the idea of a sale on Twitter. Such trading arrangements, dubbed 10b5-1 plans, are designed to enable company insiders to sell based on a set schedule, price triggers or other factors without running afoul of insider trading rules.

The filings that disclosed the subsequent sales on Tuesday and Wednesday don't include the same footnotes about preset trading plans and tax withholding obligations.

Mr. Musk signaled at a September conference that he expected to exercise options in the fourth quarter, a move that would trigger what he called a huge tax liability.

He reported selling more than 900,000 shares at prices ranging from $1,135 to about $1,196 on Monday, near record highs for Tesla's stock, whose value had increased by more than 70% this year through Friday. The difference between the value when he exercised the options and the exercise price of $6.24 will be taxable income to Mr. Musk and likely a tax deduction for the company. The lower the share price goes, if he continues to exercise options, the smaller his tax bill will be.

He would also owe taxes after selling any shares that he obtains through exercising options, based on any gain realized after exercising them. Those gains are taxed as capital gains, though any quick sales would be taxed at ordinary income-tax rates because they were held for one year or less.

He also may owe significant capital-gains taxes on the shares he sold Tuesday and Wednesday at prices between $1,000 and around $1,173. It couldn't immediately be determined what the cost basis is for those shares.

The top federal tax rate on long-term capital gains is 23.8%, though Congress is considering a surtax that would raise that to 31.8% starting in 2022, giving Mr. Musk and others an incentive to sell before any such change takes effect. That same 8% tax would also apply to his exercise of options, giving him a reason to do that now.

Mr. Musk has long been reluctant to sell Tesla shares, though he has done so to pay taxes.

Tesla shares fell sharply in the wake of Mr. Musk's Saturday poll, declining about 5% to $1,162.94 at Monday's close and almost 12% on Tuesday. The stock rallied 4.3% on Wednesday, closing at $1,067.95. Shares were up more than 2% in aftermarket trading following the share-sale disclosure.

Thanks to the decline in Tesla's share price Monday, the CEO's tax bill is likely to be lower than it would have been if he had exercised his options before the poll. For example, exercising at Friday's closing price would have yielded taxable income about 5% higher.

On Saturday, Mr. Musk framed the idea of a share sale in terms of a continuing debate about how some of America's wealthiest individuals should be taxed. That prompted Senate Finance Committee Chairman Ron Wyden to say, "Whether or not the world's wealthiest man pays any taxes at all shouldn't depend on the results of a Twitter poll." The Democrat from Oregon has voiced support for a tax on billionaires that would apply to their unrealized capital gains.

The Tesla chief executive, who is compensated in stock awards and doesn't accept a cash salary from the electric-vehicle maker, faces an August deadline to convert tens of millions of stock options into shares or let them expire worthless.

Mr. Musk isn't alone in realizing some financial gains, as Tesla's stock is near record highs. Other directors, including Kimbal Musk and board chair Robyn Denholm, have unloaded more than $600 million worth of shares this year through Friday, according to data compiled by research firm Equilar Inc. Those values don't reflect any possible cost of exercising options. Kimbal Musk sold more than $100 million worth of shares on Friday, at prices above $1,200 a share, the data show.