1) Please do not execute "Indemnity" and "Affidavit" in the same stamp paper. These two are very different and though I am not sure whether execution of something like "Indemnity cum Affidavit" on a single stamp paper is wrong legally, I would strongly suggest to execute these two on different stamp papers. Further, in e-stamp papers used in N.Delhi, there is a "descripttion of document" (meaning "what" will be executed on the stamp paper) which needs to be mentioned to the stamp vendor while purchasing, and this data is printed on the e-stamp paper. This can be either "Indemnity" or "Affidavit", but not both. Make sure the stamp paper where descripttion of document is Indemnity is used for executing Indemnity only, and where descripttion of document is Affidavit is used for executing Affidavit only. If you don't do this, it will be a serious lapse.
2) Coming to your query, the answer depends on whether Mr A can be called an "interested party". This is a grey area and I cannot be 100% sure that it will be ok to execute indemnity/affidavit by Mr B and C on stamp papers bought in the name of Mr A. See my suggestion in the next point.
3) I think that "Mr A was authorised to complete all the formalities" does not apply to execution of Indemnity and Affidavits. Here all the Directors have to execute Indemnity & Affidavits separately & individually, while other formalities may be completed by Mr A alone in representative capacity. I think that why this issue is being raised by you is that you wish to make the documents perfect so that there are no problems in future. To that end, I would suggest this: buy 6 stamp papers (SP) as follows:- SP1-in the name of Mr A, "descripttion of document" is Indemnity, SP2-in the name of Mr A, "descripttion of document" is Affidavit, SP3-in the name of Mr B, "descripttion of document" is Indemnity, SP4-in the name of Mr B, "descripttion of document" is Affidavit, SP5-in the name of Mr C, "descripttion of document" is Indemnity, SP6-in the name of Mr C, "descripttion of document" is Affidavit. Execute the documents accordingly. This will be perfect and there will be no grey areas for the future.