Monthly investments to give reasonable returns often have a longer-term horizon thatn 3-5 years. It would ideally be atleast 10 years and upwards.
The concept you are looking for is a monthly SIP.
What I would suggest is split the pool of the monthly amount you can spare and then choose 2-3 investment tools.
1.Mutual funds - choose to invest in the top two funds 50% in Equity based MF (higher return, little more risk) and 50% in balanced MF(lower return, better risk profile). This would help a person with an average risk appetite for investments.
2.Choose one part of the investment to go into a debt based MF. This is a short investment that gives MORE liquidity and a better return than a fixed deposit.If the market is in for a crash, then you can withdraw this and investment as a lump sum is some equity-based MF. This is also better in the short-term when the market is scaling new heights as there would be a fear or a chance of correction(mini-crash).
3.Plan to invest the last 1/3 if possible a smaller lump sum in NSC that gets locked in for 5 years. But you can seek the help of your tax accountant to claim tax exemption for the interest earned every year, if it's shown as re-investment. This has a better safety net than the above two and better returns than fixed deposit.
There are many sites that give good details and comparison. Choose one of the top 5 funds, see the CRISIL ranking and the returns by 3-5 years.
As a fresher, there are financial planners who advise you for a small fee, a tailor-made plan which gives the best growth for your X amount invested and the time horizon. If you set up the SIP and use them, they usually waive off the charges as they get paid by the fund houses. Once you start, you can setup a 6 month or sos review of the invest plan if the market changes or to assess is the money growing as expected or we need to change.
The bottom line is there is no single option, choose well, spread the risk, have one arm that is safe, in the event of a crash or an underperforming market at the time you can to take the money out.