You need square brackets around the references to [message]
filter {
if [target_index] == "myindex" and
("str1" in [message]
or "str2" in [message]
or "str3" in [message]){
mutate {
add_tag => ["mytag"]
}
}
That said, I would write this using alternation in a regexp
if [target_index] == "myindex" and [message] =~ /str1|str2|str3/
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