you have to make application under order 1 Rule 10 for adding parties as defendants 

Order 1, Rule 10(2): The court may at any stage of the proceedings, either upon or without the application of either party, and on such terms as may appear to the court to be just, order that the name of any party improperly joined, whether as plaintiff or defendant, be struck out, and that the name of any person who ought to have been joined, whether as plaintiff or defendant, or whose presence before the court may be necessary in order to enable the court effectually and completely to adjudicate upon and settle all the questions involved in the suit be added".

2) then apply to amend plaint 

Have to file application under O 1 R 3. Rule 10 application if file by parties who wrongly impleaded or not impleaded in a suit and not by plaintiff.

On court permitting application under O1 R3, only amended title will submit and not application of amendment.

TO correct typographical errors, both application can file simultaneously.

Move only one application under o 6 r 17 that is sufficient as you have to give details of the amendments have to be made by you including the array of parties, move immediately.

order 1 Rule 3 

swar Bhai C. Patel @ Bachu Bhai Patel vs. Harihar Behera&Anr.(16.03.1999 – SC),where the Supreme Court observed that:“This Rule requires all persons to be joined as defendants in a suit against whom any right to relief exists provided that such right is based on the same act or transaction or series of acts or transactions against those persons whether jointly, severally or in the alternative. The additional factor is that if separate suits were brought against such persons, common questions of law or fact would arise. The purpose of the Rule is to avoid multiplicity of suits”

2) you have already filed suit and you want to add party as defendant then application has to be made under Order 1 Rule 10 of cpc 

Corrigendum,

Both plaintiff and defendant can file application under O 1 R 10 and also any third party who may affect by suit.

Order1 rule 10 will be applicable

1. The petition for addition of party is to be made under order 1 rule 10 of cpc.

2. Even if you have wrongly mentioned the relevant rue then also the petition is maintainable if the content of the petition and the prayer sought for is properly mentioned.

3. There is no applicability of order 1 rule 3 of cpc.

You should make the application

under order 1 Rule 10

1 Rule 10(2) C.P.C., covers two types of cases (a) of a party who ought to have been joined but not joined and is a necessary party, and (b) of a party without whose presence the question involved in the case cannot be completely decided.

O 1 Rule 3: The simple principle is that a person is made a party in a suit because there is a cause of action against him and when causes of action are joined, the parties are also joined.

1.  Based on the above explanation, you have to ascertain for what reason do you want add more defendants, accordingly you may file a petition before the court.

2. If you want to carry out any amendments, then you may wait for this petition to be allowed after which you can file a petition seeking permission of court to make amendments not only on this regard but also to the errors that are to be rectified.

Your understanding is correct.

Thus you can decide further course of action as per prevailing circumstances.

1. As per the information mentioned in the present query, makes it clear that you want clarity on the best suitable provision of law.
2. You wish to add more parties in be suit which would be the amendments in the memo of parties and hence, order 1 rule 10 would go for you for impleading the remaining one as necessary and proper party.
3. Amendment to the pleadings will be subject to the discretion of the court as per Ofer 6 rule 17 of CPC.