Top tax rates can be found here
http://www.truthandpolitics.org/top-rates.php
Unemployment rates can be found here
http://research.stlouisfed.org/fred2/data/UNRATE.txt
Lets look at the rates since the 1960s as you claimed. I will use December since the rate change happens during the year.
1964 - drop in top rate.. 1967 .. drop in unemployment
1965 rate drop 1968 unemployment drop
1968 rate drop 1971 unemployment increase
1970 rate drop 1973 drop in unemployment
1971 rate drop 1974 increase
1981 drop 1984 drop
1982 drop 1985 drop
1987 drop 1990 drop
1988 drop 1991 increase
So we have 9 drops in top rate with 6 drops and 3 increases. (Some of the drops were pretty small) Not exactly a ringing endorsement for your every 3 years. Some correlation perhaps but statistically hard to judge.
Lets look at the increases in the top rates now
1968 increase 1971 increase
1969 increase 1970 increase
1991 increase 1994 decrease
1993 increase 1997 decrease
There appears to be no correlation when it comes to increases.
Not really enough there to make your claim ican. Some of the drops in the top rate occurred when unemployment rates were uncharactistically high.