My query is regarding the last step which package should be used to install the agent once the other steps have been followed. Currently running agent 2.2.8 on Raspberry Pi. Ideally would like to run v3.

Do I understand correctly, that you are running NetXMS server on raspberry Pi? A few questions on that matter, would be good if you can provide some info.
How reliable that works? Do you monitor many devices? What database are you using? Is the database working in raspbery Pi? Is raspberry running from SD card or it has SSD connected?

Currently the only way to get netxms on raspberry is to compile yourself. Below is instruction I used to compile netxms agent, to compile the server you need to modify ./configure command.

apt update
apt install automake libssl-dev libcurl4-openssl-dev libssh-dev libmosquitto-dev libpcre3-dev postgresql-client libpq-dev unixodbc-dev

PCRE:
wget https://ftp.pcre.org/pub/pcre/pcre-8.43.tar.gz
tar zxvf pcre-8.43.tar.gz pcre-8.43/
cd pcre-8.43/
./configure --prefix=/opt/pcre --enable-pcre32 --disable-cpp --enable-utf --disable-stack-for-recursion --enable-unicode-properties --disable-dependency-tracking
make
make install

OpenSSL:
wget https://www.openssl.org/source/openssl-1.1.1d.tar.gz
tar zxvf openssl-1.1.1d.tar.gz
cd openssl-1.1.1d
./config --prefix=/opt/openssl no-comp threads
make
make install

bcm2835 which is needed for --with-rpi-agent
apt-get install html-xml-utils
mkdir -p bcm2835 && (wget -qO - `curl -sL http://www.airspayce.com/mikem/bcm2835 | hxnormalize -x -e | hxselect -s '\n' -c "div.textblock>p:nth-child(4)>a:nth-child(1)"` | tar xz --strip-components=1 -C bcm2835 )
cd bcm2835
./configure
make
make install

NetXMS Agent:
./configure --prefix=/opt/netxms --with-agent --with-internal-libexpat --with-internal-sqlite --with-internal-zlib --with-internal-libjansson --with-mariadb --with-odbc --with-pgsql --with-sqlite --with-openssl=/opt/openssl --with-pcre=/opt/pcre --with-rpi-agent
make
make install

I definitely recommend to upgrade your server and UI to 3.0. It will work with 2.2.8 agents on Raspberry. We have build environment for RPi mostly ready, I think RPi agent will be published as part of next release.

Best regards,
Victor