First you should check if pip3 is installed on your system . To do so run the following :
pip3 --version
if it's already installed you should get something like :
pip 18.1 from /usr/lib/python3/dist-packages/pip (python 3.7)
in this case pip3 is already installed and you don't need to do anything .
Otherwise python3-pip Package is in kali-rolling repos , And to install it you need to make sure you have kali-rolling enabled in your /etc/apt/sources.list , by running following in your terminal :
echo "deb http://http.kali.org/kali kali-rolling main non-free contrib" | sudo tee /etc/apt/sources.list
apt update && apt upgrade
then finally you can install python3-pip by running :
apt install python3-pip
This should get python3-pip installed on your kali machine
If for some weird reason this didn't work for you and you needed a quick way to install pip3 on your machine you can download this file : get-pip
Then run this in your terminal :
cd ~/Downloads
python3 get-pip.py
This should get pip3 installed on your machine , you can check by running pip3 --version