Related to the monitors directly? It's tough to say, but even a 4K 27-30" monitor shouldn't be more than 70W a piece.
Assuming 70W x 3 = 210W
x24 hours a day
=5,040 Watt hours per day
/1000 = 5.04 kWh per day
5.04 kWh x 365 days a year = 1,840 kWh per year
Cost per kWh will vary depending on your locale, and there will be provided fees etc on top.
A common cost per kWh would be somewhere around 0.10-0.20 (cents, pence etc).
So, 1840 kWh x 0.1 = $184 (dollars/pounds/euros or equilvalent)
1840 kWh x 0.2 = $368
+ Fees, annually.
So your monitors themselves are going to run you somewhere in the $15-$30 per month........ *IF* you ran them at their full power (ie always on), 24/7 x 365.
So, yeah. <$10/month easily for 'normal' use.