What happens when two transactions send to the same address? I would like to know the consequences of sending bitcoin several times to the same address. Let us say I send 1 BTC from Address1 to Address3 and later I send 2 BTC from Address2 to Address3 again.
The blockchain will have both, 1 BTC and 2 BTC transactions registered. So, What hapens when I want to send 2.5 NTC from Address3 to a new address?
It works just fine.

Bitcoin transactions can have multiple inputs and outputs; each input is a "pointer" to an output of a previous transaction (specifically, it consists of a transaction id and output index). So you  would create a new transaction with two inputs:


the A1->A3 transaction (or more properly, the particular output of this transaction that credited A3)
the A2->A3 transaction (again, the particular output).


It's not necessary to include amounts in the inputs, since the rule is that each input must be consumed completely.  So we now have 3 BTC worth of inputs in our transaction. 

An output consists of an amount and a destination address. (One can also have a script which directs the payment in more elaborate ways, requiring multiple signatures, etc, but let's ignore that for now.)  You would want to have two outputs:


An output paying 2.5 BTC to the desired destination address, call it A4
An output paying the "change", 0.5 BTC, to some address of yours.  You could use A3 again, but it is generally better to use a newly generated address; call it A5.


(Actually, in real life, the second output would more likely be something like 0.4995 BTC.  The leftover 0.0005 BTC wouldn't appear in any output; it would be a transaction fee, collected by whichever miner includes this transaction in a block.)

For an end user, this is all done under the hood by their wallet software.  The user just specifies that they want to pay 2.5 BTS to A4, and the software takes care of identifying appropriate transactions to use as inputs and creating the necessary change output.  Sometimes this confuses people when they see that after the transaction, the balance in A3 has dropped from 3 to 0, if they don't know that the remaining 0.5 BTC has gone to a new address A5 which is still theirs.