MongoDB seeding and replication - node.js

Question

I have this small project i am working on it but i got stuck with mongodb replicaset and reverse proxy. I explain myself better.
I have a Web application running one instance(just a simple web page). And this application by default connects to port 3000. I used nginx reverse proxy to use port 80 instead of 3000 (and this works)
This application is a nodejs app, and in its configuration files, it should connect to mongodb on port on
mongodb://10.0.1.100:27017/posts (value exported by code into my instance)
to display some random posts.
On my mongod.conf I bind my IP to 0.0.0.0 and set port to 27017
but when i log on my app instance and seed my mongoDB (it get seeded succesfully) but when i run
npm start
I got an error that the port 3000 is already used by another service.
So, my point is, i want to connect to my /posts from my web page, without having to type :/posts
this is my mongod.conf:
# mongod.conf
# for documentation of all options, see:
# http://docs.mongodb.org/manual/reference/configuration-options/
# Where and how to store data.
storage:
dbPath: /var/lib/mongodb
journal:
enabled: true
# engine:
# mmapv1:
# wiredTiger:
# where to write logging data.
systemLog:
destination: file
logAppend: true
path: /var/log/mongodb/mongod.log
# network interfaces
net:
port: <%= #port %>
bindIp: <%= #bind_ip %>
#processManagement:
#security:
#operationProfiling:
#replication:
#sharding:
## Enterprise-Only Options:
#auditLog:
#snmp:
and in the attribute i set:
default['mongodb']['port'] = 27017
default['mongodb']['bind_ip'] = '0.0.0.0'
Question Bonus:
mongodb://10.0.1.100:27017/posts
This value has been exported in the envroment variables, but as i need to set a replication over 2 more mongodb, i tried to insert in line separated but commas, but i get an error. is there a way how to export multiple env value under one variable name?
I hope i explained myself clearly and thank you in advance guys.