Is this what you want?
sigfig <- function(x, dig = 3){
gsub("\\.$", "", formatC(signif(x,digits=dig), digits=dig, format="fg", flag="#", big.mark = ","))
}
sigfig(c(0.3459215,0.6227059,7.409618,462.3468600,9.090436,6293.4189000))
[1] "0.346" "0.623" "7.41" "462" "9.09" "6,290"