A reasonable level of margin for me is about 1.25x in my margin accounts. You should read about "margin calls" to get the basic idea - your broker can take control of your account, sell everything, and use that money to pay off the margin loan. It's part of the margin loan agreement (and IBKR is even more aggressive, monitoring & possibly selling in real time).
Previously Interactive Brokers (IBKR) had the lowest margin rates, so I assume that's still the case. Let's say you have $100k and borrow $20k on margin, paying 2.83%. Ignoring compounding, that's $566/yr. But what if you invest $10k in a 3x ETF, turning $10k into $30k invested, and also giving you $20k extra invested. Those funds charge about 1% of $30k or $300/year. In my view it's cheaper to allocate some money to a 3x ETF than go on margin.