ggplot2 heatmap with fixed scale colorbar between graphs

You need to set the same range (limits of color bar) for all of them and also specify the colors.

rng = range(matrixA, matrixB, matrixC)

And add this to your ggplot code:

g + scale_fill_gradient2(low="green", mid="lightblue", high="red", #colors in the scale
               midpoint=mean(rng),    #same midpoint for plots (mean of the range)
               breaks=seq(0,1,0.25), #breaks in the scale bar
               limits=c(floor(rng[1]), ceiling(rng[2])))

Example:

Below is an example that helps you to get what you want:

x <- matrix(60:85, 5)/100
y <- matrix(65:95, 5)/100
z <- matrix(50:100, 5)/100


rng = range(c((x), (y), (z)))

library(reshape)
library(ggplot2)

ggplot(data = melt(x)) + geom_tile(aes(x=X1,y=X2,fill = value)) +
  scale_fill_gradient2(low="green", mid="lightblue", high="red", #colors in the scale
                   midpoint=mean(rng),    #same midpoint for plots (mean of the range)
                   breaks=seq(0,1,0.25), #breaks in the scale bar
                   limits=c(floor(rng[1]), ceiling(rng[2]))) + #same limits for plots
                   ggtitle("X")

ggplot(data = melt(y)) + geom_tile(aes(x=X1,y=X2,fill = value)) +
  scale_fill_gradient2(low="green", mid="lightblue", high="red", 
                   midpoint=mean(rng),   
                   breaks=seq(0,1,0.25), 
                   limits=c(floor(rng[1]), ceiling(rng[2]))) +
                    ggtitle("Y")                  
  
ggplot(data = melt(z)) + geom_tile(aes(x=X1,y=X2,fill = value)) +
 scale_fill_gradient2(low="green", mid="lightblue", high="red", 
                   midpoint=mean(rng),    
                   breaks=seq(0,1,0.25), 
                   limits=c(floor(rng[1]), ceiling(rng[2]))) +
                    ggtitle("Z")   

This will give you: