Turn the prices of your 3 runners into market values (MV =1/PRICE)
Total the 3 market values
Distribute your bet according to the inverse proportion of each MV against the total MV.

Example of a $10 bet to cover 3 runners -- A $2.50 B$3.50 C $5.00

MV of A= 1/2.5 = 0.4
MV of B = 1/3.5 =0.285 (round up to 0.3)
MV of C= 1/5=0.2
Total MV= 0.9

Units on A = 0.4/.0.9 x10 = $4.44
Units on B= 0.3/0.9 x 10 =$ 3.33
Units on C= 0.2/0.9x 10= $2.22

Round off odd cents to nearest dollar as required. Suggest round off higher to compensate for shortening prices.

Put the basic formulae into a calculator or spreadsheet and punch in total bet amount and runner prices when needed.