$>cat junk.txt
This is a junk text
$>cat lines.txt
Line1
Line2
Line3
Line4

sed -i -e  "/Line3/r junk.txt" -e "s///" lines.txt

Line1
Line2

This is a junk text
Line4

sed -i -e '/Line3/r junk.txt' -e 's/Line3//' lines.txt

/expression/ {
    s//replacement/
    # other code here
}