Vanguard not only charges $0/trade on Vanguard ETFs, but last year they started charging $0/trade for ~1800 ETFs. So right now you can trade Schwab, iShares, SPDR or other ETFs at no cost.
https://vanguard.com/etfscommissionfreeThat's regardless of how often you buy or sell. The only restriction is SEC regulation T, which says you can't be a freeloader. You can't buy a stock with proceeds from another sale, and then sell that stock before the settlement date. You can't own an ETF for 1-2 days and sell with "unsettled funds", because there's an SEC regulation against that practice. But it's still $0/trade, even then.
I have not tried to buy 3X leveraged ETFs, but owing to my momentum experiment I discovered that exchange traded notes (ETNs) are not $0/trade. But all the other ETFs I've purchased have been $0/trade since the announcement, with no restrictions on the timing.
By the way, could you be confusing Fidelity mutual funds with Fidelity ETFs? Fidelity can't control when people buy and sell it's ETFs, because they trade on the open market. I could buy one today and sell it next week, and Fidelity can't stop it. But Fidelity mutual funds are a different matter, and they can impose restrictions on the frequency of buying and selling.