You can buy Japan 3 Year Government Bonds through brokers — choose the one that suits your needs and go ahead. You can also purchase bonds directly from the issuing organization.
The current yield rate is 0.329% — it's increased by 5.45% over the past week.
Maturity date is when a debt comes due and all principal and/or interest must be repaid to creditors. For example, the Japan 3 Year Government Bonds maturity date is Jun 20, 2027.