how to set space between buttons on Android?

In addition to the buttons you will need to use an view "empty" (1) to represent the spaces before, between and after each button.

layout_weight attribute is used to scale the spaces equally.

<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:orientation="horizontal"
    android:layout_marginTop="15dp">

    <View
        android:layout_width="0dp"
        android:layout_height="1dp"
        android:layout_weight="1"/>

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="texto1"
        android:id="@+id/btn1"
        android:onClick="botao1"
        android:textSize="10dp" />

    <View
        android:layout_width="0dp"
        android:layout_height="1dp"
        android:layout_weight="1"/>

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="texto2"
        android:id="@+id/btn2"
        android:onClick="botao2"
        android:textSize="10dp" />

    <View
        android:layout_width="0dp"
        android:layout_height="1dp"
        android:layout_weight="1"/>

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="texto3"
        android:id="@+id/btn3"
        android:onClick="botao3"
        android:textSize="10dp" />

    <View
        android:layout_width="0dp"
        android:layout_height="1dp"
        android:layout_weight="1"/>
</LinearLayout>

(1) If minSdkVersion is equal to 14 or greater you can replace

<View
    android:layout_width="0dp"
    android:layout_height="1dp"
    android:layout_weight="1"/>

by:

<Space
    android:layout_weight="1"
    android:layout_height="wrap_content"
    android:layout_width="wrap_content"/>

To add spacing between the buttons you can use layout_margin :

android:layout_margin="10dp" 
android:layout_marginTop="10dp"
android:layout_marginBottom="10dp"
android:layout_marginRight="10dp"
android:layout_marginLeft="10dp"

However, there are several ways for them to be equally spaced and allied to the user's screen.

One way: Center the Linearlayout elements and place margins on the second button

<LinearLayout
        ...
        android:gravity="center">
        ...
        <Button
            ...
            android:id="@+id/btn2"
            android:layout_marginLeft="15dp"
            android:layout_marginRight="15dp">

Seethatthedistancebetweenthebuttonsisthesame-15dp-butitisnotthesamedistancebetweenthesidesofthecellphone.

Anotherway:
Use layout_weight to set how many% the element will go occupy the parent element. (In this case 1/3 = 0.33333 ...)
Note that I used a LinearLayout as an intermediary so that the button would not be stretched, but LinearLayout.

<LinearLayout
        android:orientation="horizontal"
        android:layout_width="match_parent"
        android:layout_height="wrap_content">

        <LinearLayout
            android:orientation="vertical"
            android:layout_width="wrap_content"
            android:layout_height="match_parent"
            android:gravity="center"
            android:layout_weight=".333333333">

            <Button
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="texto1"
                android:id="@+id/btn1"
                android:onClick="botao1"
                android:textSize="10dp"/>
        </LinearLayout>

        <LinearLayout
            android:orientation="vertical"
            android:layout_width="wrap_content"
            android:layout_height="match_parent"
            android:gravity="center"
            android:layout_weight=".333333333">

            <Button
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="texto2"
                android:id="@+id/btn2"
                android:onClick="botao2"
                android:textSize="10dp"/>
        </LinearLayout>

        <LinearLayout
            android:orientation="vertical"
            android:layout_width="wrap_content"
            android:layout_height="match_parent"
            android:gravity="center"
            android:layout_weight=".333333333">

            <Button
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="texto3"
                android:id="@+id/btn3"
                android:onClick="botao3"
                android:textSize="10dp"/>
        </LinearLayout>