Assuming your filenames are accurate, so client_cert.pem and client_prv_key.pem actually contain the client cert and privatekey respectively in PEM format:

openssl pkcs12 -export -in client_cert.pem -inkey client_prv_key.pem -certfile root_cert.pem -out client.p12
# prompts for the input-key passphrase, then the output passphrase (twice)

# to specify passphrase on commandline for client_prv_key -passin pass:xyz
# similarly passphrase on commandline for the output -passout pass:xyz
# however these make the password(s) visible in ps or similar on most OSes,
# and usually visible in your shell (or CMD) history, which is often insecure

Alternatively you can provide them all on standard input in correct order:

cat client_prv_key.pem client_cert.pem root_cert.pem | openssl pkcs12 -export -out client.p12
# same options for passwords

Assuming your filenames are accurate, so client_cert.pem and client_prv_key.pem actually contain the client cert and privatekey respectively in PEM format:

openssl pkcs12 -export -in client_cert.pem -inkey client_prv_key.pem -certfile root_cert.pem -out client.p12
# prompts for the input-key passphrase, then the output passphrase (twice)

# to specify passphrase on commandline for client_prv_key -passin pass:xyz
# similarly passphrase on commandline for the output -passout pass:xyz
# however these make the password(s) visible in ps or similar on most OSes,
# and usually visible in your shell (or CMD) history, which is often insecure

Alternatively you can provide them all on standard input in correct order:

cat client_prv_key.pem client_cert.pem root_cert.pem | openssl pkcs12 -export -out client.p12
# same options for passphrase

PS- do NOT enter an empty passphrase for the PKCS12. OpenSSL lets you create it even though it's silly, but Java can't decrypt it which defeats your purpose.