Partition suit:should defendant act to implead parties even if plaintiff won't?

It's the case of plaintiff whereas you are defending your own right as defendant, accordingly the plaintiff may or may not implead third party in the suit is not your call 

A defendant can implead a subsequent purchaser as a party to a suit if the court considers it necessary or proper. This can happen in cases for specific performance or to avoid multiple lawsuits. 

If the subsequent purchasers are not made parties, it would result in further complexity in the matter since a simple partition suit has been unnecessarily complicated by the defendants' actions. These subsequent purchasers are proper parties, though not necessarily indispensable

In a partition suit, a defendant can implead another defendant if the court deems their presence necessary to effectively adjudicate the case, typically using Order 1 Rule 10 of the Civil Procedure Code (CPC) which allows for impleading parties at any stage of the proceedings if their presence is crucial for a complete resolution of the dispute; this means a defendant can bring in another party who has a stake in the property being partitioned.

You must do RIGHT NOW. Not later. 

Ideally, the plaintiff should implead the third party. But, if the plaintiff is not doing it, even a defendant can implead a third party as a defendant, because this is a partition suit. However, you must show valid reasons to the Court for impleading the third party, otherwise your petition to implead the third party may fail. So, keep all records ready, and be clear about the reasons, before filing a petition to implead the third party. 

"I found out that 3rd party has created  false records and is claiming property" How?

Has he initiated any civil action? 

Is he in possession/occupation? How?

The query is short of material facts.

Can a defendant implead another person as defendant in the same suit ? If so, which provision ?

In a partition suit, everybody is a plaintiff. The defendants are also deemed to be plaintiffs in a partition suit. There is a Supreme Court judgment saying so. 

So, a defendant may be able to implead other persons as defendants, if there are valid reasons for impleading them. This may be possible in a partition suit alone. 

Other option is, the defendant may transpose himself as a plaintiff depending on the facts and circumstances of the case. 

Usually the plaintiff decides who can be his defendants. A plaintiff may oppose impleading of defendants against his wishes. However, even if the plaintiff opposes, the Court may use its discretion to implead other persons as defendants, if there are valid reasons. It all depends on facts and circumstances of the case. 

The main question is who is in actual possession of the property. 

If the third party who created false records is in actual possession of the property, then he will be at an advantage and the real owners will be at a disadvantage. 

There is a possibility that the third party may claim adverse possession, if he is in actual possession for more than twelve years. The law of adverse possession is complicated. 

You must try to remove him from the property before twelve years, and establish your possession with evidence. 

Even if he is possession for more than twelve years, you can still take action against him, but the chances of success will be low. 

Even though adverse possession is against ethics and morals, it is legal. Supreme Court of India urged the Parliament to modify the law and make it tough to claim adverse possession, but our dear politicians are busy playing politics. It is better to scrap the law of adverse possession. 

There are also a few judgments of Supreme Court of India, which say that the law of adverse possession prevails over other laws. If the third party claims adverse possession and if you claim that you are the real owner, the third party is likely to win, because the law of adverse possession is likely to prevail over other laws. 

For example, in this case, the doctrine of lis pendens of Section 52 of Transfer of Property Act will not come to the rescue of real owners. The law of adverse possession will prevail over the doctrine of lis pendens. Supreme Court of India said so in a couple of judgments. 

As always, it all depends on the facts and circumstances of the case. You must ensure that the real owners are in actual possession of the property. If a third party is in actual possession, necessary steps must be taken against him. Impleading him into the partition suit is a good idea, because we will know what is in his mind, whether he intends to claim adverse possession. However, impleading him into the partition suit alone may not be sufficient. You may have to take other steps such as filing another suit to get the sale deed cancelled, evicting him from the property, filing criminal complaints, police case and so on. 

If the real owners are in actual possession of the property, then there is no need to worry.