Yes this is possible.

First you have to create a sequence for last three digit. Then after make a compute field which computes the string as per your need. So follow these steps.

Create data.xml under data folder add in manifest file.

<?xml version="1.0" encoding="utf-8"?>
<odoo>
    <data noupdate="1">
        <!--
        ID Sequences
        -->
        <record id="sequence_temp+id" model="ir.sequence">
            <field name="name">Sequence</field>
            <field name="code">your model name</field>
            <field name="prefix"></field>
            <field eval="1" name="number_next"/>
            <field eval="1" name="number_increment"/>
            <field eval="False" name="company_id"/>
            <field name="padding">3</field>
        </record>
    </data>
</odoo>

model.py

seq_id = fields.Char(string="string"  help="ID used for employee identification.",
                      default=lambda self: self.env['ir.sequence'].next_by_code('your model name'), invisible=True, copy=False)
computed_field = fields.Char(compute="_compute_id_with_month_year")

import datetime
current_month = datetime.datetime.today().month  # 5 eg

@api.depends('seq_id')
def _compute_id_with_month_year(self):
    for rec in self:
        if rec.seq_id:
            rec.computed_field = 'RA' + str(current_month) + rec.seq_id
        else:
            pass

Replace the string, fields names with your own needs.

Thanks