You can find FDIC insured bank accounts at 1.25% nowadays. There's also I-bonds at 1.9% inflation-linked, with a purchase limit of $10k/yr and a 1-year redemption lock-out.

3% is more accurate for the cost of margin loans or the opportunity cost of pre-paying a mortgage.

yah, 3% is a high assumption for the risk-free rate. but if it were 0%, you'd be losing money to inflation over time (treasuries are only proxies for the theoretical construct that is the risk-free rate).

no single security provides the risk-free rate but perhaps a (small) portfolio of securities could get you the risk-free return.