As markets continued to rise in April, assets held in exchange-traded funds increased for the third consecutive month.

According to monthly data from State Street Global Advisors [STT] released Monday, assets held in U.S. exchange-traded funds rose 2.9% from a month earlier to $830 billion. Exchange-traded funds outpaced the Standard & Poors 500 Composite Index, which rose 1.5% in April

Gains were evenly distributed across all 12 ETF categories, with six gaining more than $2 billion in assets. Small-cap ETFs had the largest gain, climbing $2.7 billion, followed by mid-cap ETFs, which gained $1.7 billion.

As of April 30, there were 888 ETFs managed by 31 investment managers nationally.

The top three ETF managers collective accounted for 84% of the U.S. listed ETF assets, down 0.3 percentage points since the end of last year. BlackRock [BLK], which bought the iShares family of funds from Barclays, managed 47.7% of assets, State Street had 22.9% share, and Vanguard 13.1%.

Vanguard announced last week that it reduced commissions associated with its exchange-traded funds. The Valley Forge, Pa.-based company said it would offer commission-free trading to its brokerage clients that use its lineups of 46 proprietary ETFs. The company will also offer its customers $7 trades on stocks and $2 trades on non-proprietary ETFs.

Vanguard’s move comes just a few months after Charles Schwab [SCHW] reduced commissions on online equity and non-Schwab ETF trades to $8.95. Schwab offered commission-free trading when it launched its proprietary ETF family last year.

Vanguard’s ETF assets have more than doubled to over $100 billion in the past year. The company offers ETFs that track an array of domestic and international benchmarks, including fixed income funds.

But competition is increasing. In March, JPMorgan Chase & Co. [JPM] announced in a pair of filings with the Securities and Exchange Commission that it plans to introduce a pair of exchange-traded funds. The New York-based company wants to introduce both actively managed and index-based ETFs.