HELP - Historical data, calculate lay odds from back odds 3 or more selections

[jer]

Betfair Historical data files, comes only with back prices.
For me it's ok with markets of 2 selections,ex:

Market Under/Over 2.5
Given
Under Back Price 1.73 ---> Under Lay Price ???
Over Back Price 2.26

I can calculate "Under Lay Price" inverting the "Over Back Price 2.26"

(1/2.26)*100 = 44.25% ---> 100% - 44.25% = 55.75% ---> 100/55.75 = 1.79 Done

But when more than 3 selections I can't calculate so far. Ex:

Only Back Prices given from Betfair Historical data files

Home 2.20 Away 3.50 Draw 3.30
or
Home 45.46% Away 28.57% Draw 30.30%

Now how to calculate the Lay prices based on the back prices?


Thanks in advance!

PS: The result Lay Prices I know they are Home 2.38, Away 3.9 and Draw 3.6.