If you're going the patchwork route here, then it's really not an issue of R Markdown. You'll be able to render any patchwork object you can create in R to PPT.
As far as patchwork plot layouts, there are wonderful vignettes on the patchwork website that I would encourage you to explore.
Here for example are three plots with one block of text. I'm using the gridtext package to insert the text. Check out the documentation of gridtext to see the options for text rendering -- it's quite flexible.
library(patchwork)
library(ggplot2)
library(gridtext)
point <- ggplot(iris, aes(Sepal.Length, Petal.Length)) +
geom_point()
bar <- ggplot(iris, aes(Petal.Width)) +
geom_histogram()
box <- ggplot(iris, aes(Species, Sepal.Width)) +
geom_boxplot()
text <- richtext_grob("These are my 3 plots!!")
(point + bar) / (box + text)
#> `stat_bin()` using `bins = 30`. Pick better value with `binwidth`.

Created on 2020-03-03 by the reprex package (v0.3.0)