trihybrid cross constructing a map of the genes

On chromosome #2 of the fruit fly there are three linked genes: vestigial, black, brown or bl-vg-br.  bl is in the middle.  In my parental cross there a vestigial black brown fly is mated with a wild type fly.  All F1 are wild type.  The F2 are the following:
1675 +++
23 bl vg br
176 bl ++
13 + vg br
57 ++ br
52 bl vg +
93 + vg +
17 bl + br
Based on this data, could you help me construct a map of the genes, in particular the map distances. 
I know these are linked because the data are consistent with a 3:1 ratio. 

Is the following analysis correct?
Recombinant frequency between bl and vg = 176+13+93+17 = 299 / 2106
                                                                                                  = 0.141
Recombinant frequency between vg and br = 57+52+93+17 = 219 / 2106
                                                                                                 = 0.103
Recombinant frequency between bl and br = 176+13+57+52 = 298 / 2106
                                                                                                 = 0.141

if the three calculations are correct, how can I figure out the map distances between these genes?