Minh has just received an inheritance from her grandfather$$s estate. She will be entering college in $3$ years and wants to immediately purchase three compound interest investment certificates having the following maturity values and dates: $$5000$ at the beginning of her first academic year, $$6000$ at the beginning of her second year, and $$7000$ at the beginning of her third year. She can obtain interest rates of $6\%$ compounded semiannually for any terms between three and five years, and $6\mathrm{.}6\%$ compounded quarterly for terms between five and seven years. What principal amount should she invest in each certificate?