Try this .

Its called.
Betting to Price.

Divide bank by 20
e.g. 300.00 / 20 = 15 This is our Base Ret figure

Now divide the price of your selection into this figure.
15.00 / 3.00 = O/L5.00
15.00 / 5.00 = O/L3.00
15.00 / 10.00 = O/L 1.50
Total O/L 9.50
Bet to Ret 15.00

Profit if any one wins 5.50

All separate races or the same race.

If any get up , we are in profit, even if the 3.00 shot gets up, where at level stakes , one would have broke even.

What you will see is that we are placing more on the shorter priced horses & less on the longer priced horses.

It works the best on your own price ratings & if they are OK , one should do well.

This method usually shows better returns than just level stakes betting.

Another way of using this is in that example you gave us.
If the market says its 3.00 in a maiden race & has never run before & feel it should be priced at 4.00 then that's the price you divide into our 15.00 base ret figure.

That way , if it falls over , we didn't risk as much.

This approach keeps the Risk factor (Liability) totally under control.
Which is half of what the punt is about.

You can even do this with say 2-4 horses in the same race if prices permit.

This is the approach bookies use when they are betting with other bookies , say 3 horses in the same race.

The reason way it works is that the majority of a punters winners tend to be the shorter prices in the majority of times not the longer prices.

But if one feels they are the exception to the rule , put more on the long prices & less on the short prices .
I have a feeling you already know what will happen if one tries that long term..

Cheers.

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Cheers.